from what i remember (i barely looked at the website) you don't submerge as much as you hop and dive under for a little bit. Like how you can make a jet ski hop by bouncing on it, you can control this to hop and then dive under for a quick underwater frolic.
The one in the link isn't the one I had initially read about. The newer model has a 250HP supercharged Atkins motor. It'll do 50mph on top and 30mph under water.
Says it'll only dive 5 feet because the motor starts to choke out...automatic closing snorkel...