Flip-flops and Crocs definitely get really slippery when wet...and you are right that water shoes are sooooo hard to get on and off when they are wet!
Those Speedo water shoes look like they might be a good choice, and they're cute!
I personally use flip-flops at the water parks, but I don't wear them into the wave pool, keeping them as dry as possible. Try to find flip-flops that don't have a glossy finish to them (i.e. Old Navy is usually good; avoid Wal-Mart) because that helps cut down on the slickness. There is nothing easier to take off and hold onto than flip-flops. They just slip right off, and you can hold them against your chest on body slides or loop a finger through the straps while on tube slides...that still leaves your hands free to grip the tube handles.
You could also consider bringing water shoes for the wave pool and flip-flops for other parts of the park, if you're willing to leave one or the other by your chair and/or in your locker.
Hope that helps!
