You can get pretty wet on the river rapids ride....but I think that is the only water ride in the park. There are a couple of splash areas for kids (in bug's land and nearby in the farmer's market area). My DH and DS bring clothes to change into, but we go in the winter when the weather is cooler........
DTDkettleCORNfan is correct. The only water ride in DCA is Grizzly River Run, but you can (and most likely will) get soaked. Depending on the time of year, you may want a change of clothes or try wearing a poncho on the ride.
Even if you DO wear a poncho, if you don't wear the hood you will end up possibly soaked on the top b/c the water got in the neckhole. And if you're bringing bags on the ride and protecting them under your poncho, your legs might end up soaked.
I know that last bit b/c I did that on my first two rides, when I could not find the FREE lockers.
So if you are carrying stuff that shouldn't get wet, go to the lockers behind the front of the line (looking at the front, it's to the left, over around behind the big bear). It gives you approximately 2 hours of FREE secure storage for your stuff.
So if I were him I'd buy a poncho or wear something quick-drying (my hubby was so hot in May that he went on Grizzly in order to get soaked, he felt refreshed as he dried, LOL), or if you guys have reservations somewhere nice, maybe then a change of clothes.