One thing that I didn't like in Halo is that you drowned instantly in water( besides in Halo 1, you could drive vehicles in water) but I hope that in Destiny that you could go underwater and explore reefs and look at fish, that would be very exciting because I love fish and coral so I would be underwater most of the time and explore the water.
I would love to have sharks in the ocean as well so that I could avenge my grandpa who was killed by Tiger Sharks which are know for purposefully killing and eating humans. :-(
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Doubt it, as big and open as the world looks, its still a limited sandbox, like Borderlands, so the water is one of many natural elements used to contain players (and honestly, I rather have my character drown then awkwardly bumping into a invisible wall)