I've been taking my baby to the pool since he was two months old. He loves it! There are actually special diapers available from pampers called "lil swimmers" or something like that. They act kinda like pull ups and tear away when you are done. You could put some shorts on or a onsie over it. If he does poop, you will have to take him out because I'm not sure how well it will contain THAT type of accident
Now if you want to use a floatation device, there are these things available that look like a cross between a life jacket and a wet suit. They work GREAT! I am a little wary of any other type, like intertubes, as a baby can fli in them or get trapped. The arm floaties are more for older kids. Sometimes I just hold alex and swim around with him, but as he gets older and stronger, I will get him one of those baby life jackets
Have fun with your baby! I am sur eyou both will love it!