I'm considering joining the military, could i get info/advice/opinions?

Ok, well i'm trying to figure out which branch. I've put deep thought into the marines, i'm still undecided. I couldn't do navy because i have somewhat of a phobia of large bodies of water, being on a boats fine, but i don't think i could dive in the ocean. I'd love to do the airforce, but i've heard they're picky and they're more brainy than the rest of the branch...to be be honest, i'm not that sharp. The army sounds more of my field....how misinformed am i? What should i expect if i join?...keep in mind i'm 20 turning 21 next month