When Saving in Water, the game acts strange and visually, has a problem

My Minecraft lags a lot!! But this was weird. I decided to try Survival, since i was looking at servers to play but never tried...So i went on singleplayer, and made a Survival map for myself to practice. I spawned in a area surrounded by water, and there was two islands, and i spawned a chest in the stages, where you get a chest in survival. Then I went to the other island and collected the wood. then started swimming. Well, i had to go do chores, and while in water, at night, i decide (and forget to just pause) to save and quit to the main menu. Then when i got back, i went back on Minecraft, and went back to my "server." I got back, and the whole place was black, pitch black, i spawned in the water, drowning, and i couldnt get back up. So, I go back to the main menu, and make another "server." Problem is, that i spawned, in pitch black, by vines, i could see the vines, and the area, where you hit a block, the square on the srceen was surrounded in white, and much more lag. Please, i think i just broke Minecraft on my PC!! Help! I love Minecraft!!