Translation:I have the swimwear, but where is the towel?
Uikkarit can be singular or plural. I'd say it could also include bikinis and the like. If someone asks you to come along for a swim, and if you have your uikkarit, you wouldn't say "no, but I have a bikini", you'd just say "yes".
(Having said that, nude swimming is not uncommon in Finland, as long as it's not in a very public place, etc. There's even an indoor pool in Helsinki which is very old and only has one big dressing room and shower area, and therefore separate male and female swimming times, and you can swim in the nude if you like. About half of the people do, in my experience. It's funny to see swimmers with a swimming cap, goggles, heartrate monitor band around the chest ... and nothing else on.)
I understand that "uikkarit ovat minulla" means "I have the swimwear," as opposed to switching the order could mean "I have a swimsuit," (not sure if my example works with swimwear but feel free to substitute a noun that is maybe more logical with singular and plural usage), but is the order of "missä pyyhe on" specifically referring to "the towel" as opposed to "a towel?" I thought "missä _ on?" could be the or a. And my otherwise correct response was not accepted as i answered "I have the swimwear, but where is a towel?"