Pressure underneath wave

Question

I enjoy water sports and activities in the water. I've swam around and dived down enough to need to equalize my ears and in general have felt water pressure.

I've since taken up surfing and have been intrigued by my perceived pressure experience underneath somewhat larger waves and was wondering how the pressure works when diving underneath waves.

When paddling out to surf is advantageous to dive underneath the breaking waves to avoid being pulled back towards the beach in the white water.

Here is an example:

Where I get confused is that I can duck dive under a 8-9' wave and not feel the pressure drastically change.

But if I just swam down 8 or 9' underwater I would definitely feel the pressure.

I've often heard when talking about water pressure something like "think of all the water directly above you pushing down" And so when going under a wave there is the same amount of water pushing down (or so I imagine)

So the question I'm looking to have answered is how pressure is calculated underneath a wave. And how this scales to really really big waves.

Answer 1

This is not really in the realm of my expertise, but I found the question interesting and had some thoughts about it:

A wave is a moving fluid. To move, the fluid is not in static, but maybe stationary equilibrium (or maybe not even in an equilibrium at all :P). To start moving, it has to be accelerated. To be accelerated, a force has to act upon it. This force is probably the pressure gradient. Now, we have a situation where gravity is balanced by a movement, or rather the acceleration by Earths gravity is balanced by an acceleration by pressure difference in the up-down direction. But now, the fluid is "deformed" into a wave. The relevant part for the pressure inside the wave is now the thickness perpendicular to the surface-normal of the wave, which is NOT THE HEIGHT of the wave!

As I said, I am no expert on fluid mechanics and this might be completely wrong.