when my son was a baby, we lived in the pool pretty much. I had a floaty that he fit in perfectly and the pool that we would go to had lap lanes, so i would put him in front of me, and kick my way down the lane holding onto his floaty. it was a great workout and it was great being face to face with him and making silly faces and playing with him that way, then i would turn him around on our way back where his back was to me and i would swim on my back with him still in his float sitting on my tummy and kick my feet through the water splashing water at him sometimes. (if that makes since) he loved it. there are so many things that you can do in the water with your little one to have fun.
as far as sunblock goes, without a doubt. i'm sure you know that though. :)
anywho, hope i helped a little.