Sorry Just realised misread part of the original question...been reading too much fantasy and things! Following on from my last thoughts a bit - Dr Who and those underwater monsters...what were they called?!
Depends how clear the water is but all sorts of things and feelings from beauty and tranquillity to sheer brutality in other cases. From kelp beds forests with sun shining through like a woodland glade to seeing hermit crabs in Cornwall and the strangeness of an underwater cave..maybe I better stop or you'll end up with a poem - lol!