I would just use your least favorite PLU diaper/ cover. No inserts. I don't think one swim will ruin it. But if it does do some harm at least it's one you don't like. (A swim diaper is just to keep solids out of the pool, so it doesn't need to be the best fit)
Re: Forgot a swim diaper
Our English Bulldog, Ulga born 4/9/2013
Baby Boy Xander born 3/13/2014
IUI#2 Femara/Ovidrel (cd 5-9) = BFN
IUI#3 Femara/Ovidrel (cd 3-7) = BFP!
beta #1 11/23 = 270, P4 = 75
beta #2 11/28 = 2055
Our daughter E was born 7/29/2012!
Surprise, our 2nd daughter P was born 5/22/14!
Our English Bulldog, Ulga born 4/9/2013
Baby Boy Xander born 3/13/2014