The real issue here is that you can swim in Ecco the Dolphin, which is easily the most similar game to Sonic Unleashed that I can think of off-hand. The fact that Ecco allowed you to swim all those years ago, and Sonic Unleashed, which is basically trying to be the exact same thing, doesn't let you do so many years later is just a joke. It's really just dissapointing.
Now you compare this to Burnout, and we can see that the water in the game is analogous to the buildings and in some cases bollards or trees surrounding the track. Now running into these does NOT result in instant death of the driver, just like it should be. In fact, the only time I can think of something like that killing you in Burnout might be during the final boss battle, but I haven't played the game for some time, so I can't really say with accuracy.
So we can see that Ecco and Burnout both allow you to swim without dying (in fact Ecco even encourages it, although I believe this was through an accident in the coding rather than an intentional game design, similar to the Hunter/Spotter relationship in Resident Evil: Code Veronica). This is good. What is not good is that God of War and Sonic the Hedgehog (1991) again, are basically the same game with similar technical prowess, but one allows you to RUN under water (Not swim) while in the other, landing in water will occasionally kill you. It seems strange at first, but when you consider that Half-Life (which actually pre-dates God of War) allows you to swim, but with the limitation of needing to come up for air. It was obvious that the developers of God of War were influenced by this and tried to come up with some form of compromise, but in a less realistic manner to fit in with the mythological/fantasy setting of the series.
At the end of the day in God of War (not Sonic Unleashed) it's a decision made because of the art of the game. Well, Art is Art, isn't it? Still, on the other hand, water is water. And east is east and west is west and if you take cranberries and stew them like applesauce they taste much more like prunes than rhubarb does. Now you tell me what you know.