Its always best to wait until theyve had all their jabs (usually around 4 months) before taking them to a public pool. When you do take him, only take in the water for 10-15 minutes to start as babies get very cold very quick. As he gets used to it and gets older, you can stay for longer.
Its best to always try and keep babies out of the sun. I know its not always easy, I myself have a 4 month old and even using her sunshade, there are times when the sun gets through the gaps. If the sun is strong, I always use a sunblock with a minimum SPF of 40. I put it on any parts that may be exposed to the sun, even if its only for a few minutes. Baby skin is so much more delicate than ours and will burn very quick. I keep a sunhat for her in her bag too for any time her head may be exposed at all.
Hope you have fun with the swimming, mine has just had her last lot of jabs so I will be taking her soon and cant wait :)