I think that the rules are different for sea creatures simply because accurate sight is usually a less useful position sense in water. In most places you can’t see far away no matter how good your eyes are, so just noticing shadows is mostly enough. Sound (including vibrations and currents) tends to be more useful there, hence echolocation and the lateral line, as is smell (see sharks). Basically, you can’t hunt much with sight, but it’s still useful to avoid being hunted.

There are some exceptions, like octopi (big eyes) and some fish with curiously complex sight (poly-chromatic, polarization-sensitive eyes) I don’t have a very good explanation for. But I’d guess they’re a bit like bats for land animals, some accident of evolution probably threw them on a tangent and they found a “local maxima” of fitness.