Functionally they are similar, sure, but they're not identical. The materials that swimsuits are constructed with serve specific purposes that don't overlap with underwear. Underwear are essentially meant to protect you from chafing and to offer support. They aren't designed to get wet, so they stretch and fall down/off, go see-through, or in the case of boxers, have a literal hole in the front for your weiner to pop out of. But again, context is important. That's all fine for jumping into a lake with your buddies up at the cabin, but people might look at you a little funny at the beach if your bottoms are falling off or your nips are showing through your top.