I think the Sea Raycers are limited to 20mph. With two people and a smooth lake, you might see something close to that. But, generally speaking, Bay Lake always has a bit of chop to it due to the ferry boat traffic. Expect 15mph to be the realistic top speed.
The Seadoo personal water craft are stuck in "learner's mode", which limits the speed to 30mph or less. And, they limit where you can go with them (which is understandable).
I haven't rented a boat at WDW since 1974 when I was 13 years old. Since then, I've handled big boats, small boats, and personal watercraft. I've owned big boats, small boats, and personal water craft. Today, if a boat can't do at least 50mph, I find it quite boring.