When going to Orlando late March/early April, we usually start out the day with a sweatshirt over a t shirt and shorts. As the day goes on, you don't need the sweatshirt. Depending on the day, you might need to put the sweatshirt back on after the sun goes down.
I agree with bellelinus. I have been watching the weather down in Orlando. We are headed down in a couple of weeks. I am dreading packing for day and evening. I think it will be necessary to dress warmer in the evenings and t shirt and shorts during the day.
the local water parks are getting ready to open, if that tells you anything about swimming. it's all about the sun here in the winter/spring. if it's out, it's perfect. if it goes under, it feels about 10 degrees colder. yesterday was supposed to be about 75, but it felt like 80. I could have worn a tank top easily. hope that adds to your confusion. a long sleeved shirt or a sweatshirt is probably a good idea for evenings the first of April.
As long as you have something to layer over the tank top, you can have your baby wear it and judge as the day goes on whether it is warm enough to take the top layer off. You can get cloudy days where you need a sweater or sweatshirt the entire day, and you can get days where it is actually hot by mid-day. As for the swimming, I think it is definitely warm enough to swim in April (but then again, my girls and I were the only ones in the pool one day between Christmas and New Year's!).