Actually, its more like indifference than hostility. You can deal with hostility by preparing yourself for combat. Indifference requires a more subtle and sophisticated response.

But yes, the ocean is like outer space, it can seem an extremely hostile environment.

If the weather is warm and gentle you can last there at most a few days without technology, if you're a strong swimmer. But most of the time, in most places, it can kill you in minutes.

When you think about it, naked, we evolved for a tropical savanna or rain forest. Without technology and companions it's about all we can handle.