I see allot of planes & trucks / cars in this thread but, I just got into Fast Electric Boats and it's a scream. The boat I have is an Aquacraft Revolt 30. It is a ready to run type and was fully assembled. Out of the box it hit 35 mph. This may not sound fast but, with a few adjustments and prop swaps I've gotten it to 47.8 mph.
The picture above shows the Velocity RC GPS showing 46.6mph. I did hit 47.8 but, the picture wasn't in focus.