our baby wears something called a 'happy nappy' over the top - some pools require this, which is a v tight pair of 'hotpants' that go over the swim nappy and is made of stretchy wetsuit type material. This holds the nappy in place. We bought ours from the swim school but a pool with a shop will prob sell them.
If we're swimming in a specially heated pool for babies/children i.e. Over 30degrees, that's all. But if in a regular pool - usually about 29 degrees, he wears a sleeveless rompersuit type wet suit over the top.
whatever he wears, for his 1st swim you probably don't want to stay in the pool for more than 10 to 15mins. Babies at that age get cold and tired v quickly in water and it's better to have a short happy swim than risk them getting upset so they learn to love water from an early age.
Well done for getting started early. The Little Dippers website (where we have lessons) has some good advice on going swimming with your baby too.
We're going for the first time tomorrow and just have some swim nappies, does my teeny tiny 7 week old son need a cozzie too to go over the top? He's still in size ones and newborn sizes and I couldn't find anything in town today.