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Many questions have been asked here about why the Earth has a magnetic field, e.g.,

At the risk of oversimplifying a bit, the answer is the dynamo theory. Convection in an electrically conductive, rotating fluid – in this case, the molten metal in the planet's core – creates electric currents that, in turn, generate a magnetic field.

Why doesn't the same thing happen in the oceans? A large ocean like the Pacific would appear to have all of the general properties required for a dynamo. It is made of conductive saltwater; it has significant bulk flows (indeed, ocean currents are much faster than convective currents in the core); and it rotates with the planet. Is the higher resistivity the key difference? If so, would a saltier ocean be able to generate a magnetic field?

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    $\begingroup$ IMHO, this question would be particularly interesting when applied to planets that have no flowing magma. $\endgroup$ Jun 16 at 0:56
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    $\begingroup$ I hypothesize that as big as the ocean seems, it is miniscule in mass compared to the core. $\endgroup$
    – DKNguyen
    Jun 16 at 0:57
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    $\begingroup$ This image from the USGS graphically shows that there's not a lot of water (relatively speaking) on the surface of the Earth (or in the upper crust). $\endgroup$
    – PM 2Ring
    Jun 16 at 2:49
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    $\begingroup$ AFAIK; The dynamo theory stipulates convection driven currents as helics arranged axially (convection drives radially, coriolis "forces" turns it axially). The oceans have a limited depth (compared to the liquid mantle) and they have a limited convection force to drive fluids outwards. I suspect the main effect would be it's thinner crust. $\endgroup$ Jun 16 at 10:10
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    $\begingroup$ A meta-question for you: I have frequently seen the pattern on SE sites of "here is an argument why X should occur; why does X not occur?" and then the answer is "your argument is correct and X does occur". It seems like this way of phrasing a question is making unnecessary work for everyone involved; is there some benefit to this style of question that I'm not seeing? $\endgroup$ Jun 16 at 23:24
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The earth's oceans do in fact generate a measurable magnetic field$^1$. As you have pointed out, the motion of charged particles generate magnetic fields, so it makes sense that the earth’s oceans would do the same. In fact, the oceans make a contribution (albeit a small one) to the Earth's overall magnetic field.

The moving salts within the oceans have electrical charge (which means you have electrical currents), and since the oceans move in cycles, the motion of the tides etc., as you pointed out, the oceans contribute to the total magnetic field of the earth.

In the following image, we see how this magnetic field is distributed about the northern hemisphere with the United States and Canada in the center of the sphere, and how its strength varies at different points. The European Space Agency in 2013 launched three satellites, a system called Swarm, which was designed to study the earth's magnetic field in detail and was also used to map the magnetic field emanating from the oceans.

In fact here

As can be seen, the ocean generated magnetic field is on average $(1 \ \text{to} \ 2)\times 10^{-9}$ Tesla at sea level. This field goes to roughly $10^{-9}$ Tesla at the height of about a few hundred kilometers, or average satellite height. This means that this magnetic field is about $20,000 \times$ smaller than the Earth's magnetic field ($\approx 40\mu$Tesla) caused by the motion of charged particles in the Earth’s core.

$^1$ Analysis of Ocean Tide-Induced Magnetic Fields

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    $\begingroup$ But there isn't any (significant) net charge (neutral molecules dissolved in water). Why don't they cancel out? Why aren't the differences, for example, due a water-depth and salinity-dependent magnetic permeability? This could be addressed in the answer. $\endgroup$ Jun 16 at 12:59
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    $\begingroup$ @PeterMortensen You don't need a net charge for a dynamo, it suffices that the medium is an electric conductor. Whenever such a medium moves perpendicular to a magnetic field, you get an inducted current. And that current has a magnetic field of its own which drives the dynamo. $\endgroup$ Jun 16 at 15:12
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    $\begingroup$ What's really interesting about this image is that it clearly shows that the ocean's magnetic tidal signal has structure that is going through the continent of North America. Since I don't believe that oceans go under the continents, the only logical source of such structure is the earth's own geomagnetic field. $\endgroup$ Jun 17 at 12:31
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As joseph h said, the oceans do have a contribution to the magnetic field, and the reason that this contribution is small is just that the oceans are very thin compared to the Earth's radius. As a result, any vertical convection currents don't have a chance to pick up enough Coriolis force to drive a self-sustaining dynamo.

There are no other bodies in the solar systems with oceans that act as full dynamos either, although especially for some Jovian moons the sub-surface oceans do have a quite marked magnetic interaction with Jupiter's magnetosphere. This is particularly notable for Europa, which does not have a liquid-core dynamo, whereas Ganymede does also have a dynamo core similar to Earth's.

The closest thing to an ocean-dynamo we have in the solar system are Uranus and Neptune. Both have significant intrisic magnetic fields (and magnetospheres), but the fields are very different from Earth's, Jupiter's or the Sun's – namely, they aren't dipoles to any reasonable approximation. The probable reason is that the active dynamo region is much closer to the “surface” than in case of Earth, and the relevant flowing material would have to be some combination of water, ammonia, and various ions.

Schilling et al. 2007: Time-varying interaction of Europa with the jovian magnetosphere: Constraints on the conductivity of Europa's subsurface ocean

Schubert et al. 1996: The magnetic field and internal structure of Ganymede

Stanley & Bloxham 2006: Numerical dynamo models of Uranus' and Neptune's magnetic fields

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Maybe it's good to point out how this field comes about. You would expect no field as the currents are both equally positively and negatively charged. On their own they don't produce a magnetic field. This only comes about if tboth currents interact with magnetic field of the Earth. These currents are separated because they move through the magnetic field. When they are separated they can produce their own local magnetic fields not canceled by the opposite current (which would be the case if they were not separated),
So if the Earth itself didn't produce a magnetic field the oceons would be neutral.

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    $\begingroup$ Is this true given @cmaster's comment on another answer? $\endgroup$ Jun 16 at 20:44
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    $\begingroup$ Indeed it's not true. There is not fundamental difference between the capability of a saltwater ocean and a molten-iron core to sustain a magnetic dynamo on their own, without any external source. All that's required is conductivity, liquidness and rotation. The reason that the oceans only contribute a small part to Earth's field is that they're comparatively thin. Indeed (and only for that reason) they would not sustain a field by themselves. It so happens that no other bodies in the solar system have a thick enough ocean either, but that's circumstantial. $\endgroup$ Jun 17 at 10:56
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    $\begingroup$ @leftaroundabout I don't think it's entirely untrue-- just not conveying the whole picture. Any dynamo, while perhaps able to self-sustain, needs some external field to get it started, and this answer is about that process of getting started. I agree the answer could be improved by emphasizing the distinction between a self-sustaining dynamo like the core, for which any nontrivial perturbation (like the sun's field) or initial conditions should grow and persist, and the ocean (not self-sustaining due to its geometry, as you point out), for which a consistent external driver is needed. $\endgroup$
    – jawheele
    Jun 17 at 17:34
  • $\begingroup$ @jawheele fair enough, but the answer doesn't put it that way. $\endgroup$ Jun 17 at 18:47

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