The easiest "lens" in this case is no lens at all... it is a curved mask with the curvature centered on the optical center of your eye. This will ensure that all rays continue in a straight line towards your eye.
I explained this in more detail in this answer
If you actually want to immerse a lens in seawater to achieve the same effect, then you need to somehow overcome the fact that the interface between your cornea and the sea is less effective. In a sense, underwater you are focused "beyond infinity" and need a lens that has a focal length of about 5 cm while immersed in water. This would require a convex lens. The highest index you can "reasonably" buy is 1.74 for high-index plastic lenses (higher index materials exist but are not usually used for commercial grade lenses, and I assume you want this to be costeffective).
Now the index of refraction of sea water is about 1.34; the index of refraction of the cornea is 1.376 - very close to that of seawater. So we can assume it does not provide any focusing. The lens in the eye provides about 20% of the total refraction of the incident light (according to this), so we need about 80% of the focusing power of the eye.
Now the total length of the eyeball (according to the same link) from cornea to retina is about 24 mm. Add to this a distance of 2 cm from cornea to the lens we want to use, we need this lens to provide focusing about 44 mm - with about 20% of that provided by the lens in the eye, we need a focal length (in sea water) of about 55 mm.
With a refractive index of 1.74 in a medium of 1.34, we are at "roughly" half the refracting power that you would normally need. So a high index lens with a focal length of 55 mm in sea water would need a focal length of about 28 mm in air. That's roughly 35 diopters - more than you would "normally" get for lenses in ordinary eyeglasses.
Obviously, if you put the lens further from the eye, the required focal length is less demanding - but in order to get the same FOV you would need a correspondingly bigger lens.
Just some thoughts to get you going...