Going by Sega Technical Institute's old character outlines, Sonic loves nature in general and especially the oceans. With his lust for adventure, I imagine the risks only add to their beauty in his eyes. In other profiles /interviews, it's also been said that Sonic doesn't "see" obstacles. I figure he just sees different kinds of spring boards. If you can't swim, walk on water!

Also, he's with friends here - if he lost speed and sank, they'd pick him up. These guys were Sonic's first named friends. Though they've faded from the games, I like to think that the same way South Island is one of his favorite islands, these first friends hold a special place in his heart, and that he comes back to them, no matter how many new friends he makes around the world.

Sonic, Flicky, Pocky, Pecky, Ricky, Cucky, Rocky and Picky (c) Sega
art by me
(yes, the picture is flipped after scanning - I realized the flow worked better that way. :p )