If the swim is wetsuit legal, you can use the floatie pants. If not, you can't.

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I'd be trying a sleeveless wetsuit before to see how that goes before going just the pants as you will get a lot more advantage. I get a set of Orca 3/4 wetsuit pants I wear in the pool at times would be your best most advantage if only going shorts in wetsuit legal races.

DeSoto has a nice pair of long pants, gets you a lot of the advantage of a wetsuit in warm water. But keep in mind, anything made of neoprene is considered a wetsuit, even the jammer floatie ones, so are subject to any rules governing them. If wetsuits are allowed, then you can wear them, if not, then no..