Tesafilmklebt: I can try it with the water quality and the game never crashes while other situations
Now i can say it doesn't help to set the water quality on low :/
Does it crash as soon as you enter the water, or only after you swim around for a while? Is it kind of random? Does it happen while you dive, or also while you're just swimming on the surface? Does water in general cause these crashes, or only once you enter it?
Also, what are your GPU and CPU specs? Did you check your graphics card statistics after the crash -- anything unusual, like very high temperatures? ( Also, did you overclock your GPU? )
Is the game fully patched up, and are your graphics card drivers up to date?
So far I only encountered one crash myself, and several of those "freezes" which mostly happen in the inventory. ( But supposedly those have been fixed in recent patches. )