He says I've got about 10 minutes to shower and be ready. 

I kind of don't want to go, because I'm a little uncomfortable being in a bathingsuit around strangers, even though I like swimming. Also, if I go then it will delay my reply by several hours. 

But he'll be super-disappointed if I don't go (again, I don't HAVE to be naked, in fact I don't even have to swim), and it does sound fun.