Just back from our first Universal trip.... we were so focused on Potter that we totally forgot to plan for the water rides! Completely soaked, through the sneakers.... not good!
White t-shirt too... yikes. We bought new t's to change into and just went with it. Luckily i had a plastic bag for my phone, etc.
As for the bathing suit idea - they were definitely stopping men and women from taking off their t-shirts to ride; you have to be wearing a shirt and shorts, n
o suits alone.
On previous trips to other parks, I've worn quick-dry shorts and packed a change of undergarments and tshirt in my backpack - quite happy with that solution.