There are 2 major differences between a swimsuit and a bodysuit:
1) Whether there is the snap button crotch closure. As you know, one may wear a bodysuit all day long, so when she needs to go to the toilet, a snap button on crotch will be very convenient. So that most of bodysuits come with snap button design on the crotch.
2) Whethe there are bust pads or wires. Most of swimwears comes with bust pads or even wires, but normally a bodysuit will not be designed with bust pads.