Well, if you free range night and day, the guineas may have trouble learning to stay home. You'll want to keep them confined during brooding so they know where home is (we kept ours in the coop for six weeks).
I guess I could maybe see a six-week guinea drowning itself in a fit of foolishness. But no more so than a six-week old chicken might. On the other hand, at six weeks they can fly like the dickens, so it would probably fly right out of the water anyway.
I agree that guineas are not at all stupid, though they are very easily distracted. Teaching them to go back into the coop at night was quite the challenge.
We think ours are great. They like to follow us around and see what we're doing. They've actually turned out more interested in our activities than almost all of the chickens. I'd get them if I were you.