My first camera was a Nikonos V. If you have a deep interest in macro work, I don’t know what to tell you. However- if your interest is in taking photos of large animals: rays, sharks, divers, etc- even the most crystalline water will demand that you get well within… 7 feet? or 3 feet.
Hmmph. What’s most important for non- macro is:
a wide angle lens
a strobe or two B/C: below a few feet, the water column steals color. also, strobes will allow you to stop down to 16 or 22 and that helps when you are reaching out, literally, for images.
perhaps you knew all this.
I havent shot u/w in a long time- but Nikonos must still be the best deal. and cousteau used it.
Oh. But if you wanna shoot surf… well, the only Nikonos surface lens is 35 mm. that could be a challenge.