A one-piece fleece suit under your overall keeps you warm even when wet. It stays put and doesn't need a waistband. That makes it more comfortable.
The fleece fabric doesn't hold on to water, but runs off quickly after a swim, giving you a feeling of being warm and almost dry. This is good for many immersion sports where you're in and out of the water all the time.
You can also use a fleece suit on its own for pool swim training or just for wet fun. It soaks up a lot of water making your resistance swim training really effective. When you come out of the pool, the water runs out quickly.
A fleece overall stays put under your nylon overall. This is important when you jump about a lot, in and out of the water. Nothing is more annoying when the clothes inside your overall sag and slip around.
Fleece overall for pool training.
Loose fit for comfort in the water.