I'm not sure of the exact physics of it, but basically, you keep all or a portion of your horizontal momentum upon entering float. Therefore, to get max momentum when entering it, you need to increase your momentum before entering it. The best way to do this is with control of two aspects of Melee physics: analog jumps and momentum preservation from jump.
Analog jumps are explained in
this video by Gravy. He talks about analog jumps on the ground, but the same technique works in the air as well. When analog jumping in the air, the game reads your horizontal analog input on the same frame that you press jump. Analog jumps allow you to have a lot more momentum in the air after the jump is performed.
Another Melee physics trait is that you maintain most if not all (depending on character) of your ground momentum upon entering jumpsquat and, subsequently, the air. This is very useful for characters with fast dashes who can dash then jump to cover long distances. Peach isn't very gifted in this regard, but it's still better than jumping from standing still.
The take away from all this is that to get max momentum from float, control your speed with analog jumps and momentum preservation, especially the analog jumps. Dash->jump+full analog to the side->then back down to instant float should give you a fast float off the ground. In the air, double jump with a full analog left or right before floating to get a high speed float. This last technique is especially useful for quickly changing direction in the air. You can move forward from the air either out of a jump or off a platform, jump backwards and quickly enter float. That can trick people up and set up nicely for retreating fair.
