Lake by a waterslide. The ocean scares the absolute shit out of me. I remember jigging (putting a hook over the side of a boat and moving it up and down - kinda like fishing without the pole) with my grandfather in his little boat out on the open ocean. I don't know if it was something that crossed my mind then, or just a fear I've implanted since, but I recall thinking about what sort of huge ass creatures could be swimming beneath me in all that darkness. The concept of the giant squid was enough to terrorize me, but I've also always been destroyed by the sheer size of actual whales. The thought of a whale floating by beneath that little boat in all its random immensity...
Blarg. Lakes are fine, as long as they're not Nessy lakes with big ass dinosaurs. Though just a few years ago I heard about duck itch, which apparently is something that can happen to your skin if ducks poop in the lake you're swimming in. Weird.
Also, fishing in lakes is way more fun, on account of the impossibility of reeling in any sort of angry squid-type creature. In lakes, you usually get fish or boots.
I'd take a river over either of these, just for the record. I have a good memory of me, my dad and my uncle walking up a river doing what we called 'feeling', where you stick your hands under rocks and grab trout with your bare hands. That was kinda the tits 'cause you got to feel like a bear. The animal kind not the gay kind.
Short Answer: Lakes are beautiful places where you take vacations. The ocean is where man goes to die.