If I recall, the ocean is about a 30second swim deep.
This is not normally possible to tell because at some point the ocean becomes murky and camouflaged so you still get a swim animation, but can't tell if you're moving or not.
The method I discovered the bottom by, was to follow the sloping coast of a sea post. At depth it abruptly ended and became "deep murky sea" that I could not swim down further through. I could swim out and look at the abrupt end of the island and still could not swim further down.
Other methods might be to time your descent and compare your ascent. Swim down 45 seconds with pineapples or something. And then swim up.
Map the disparity in seconds.
I'd estimate based on this research you'll get about 30 second ascends no matter how long you swim down.
I can be off on the times. But I remember not having to take substantial damage to surface.
