Hi @djmcmath, welcome to the forum 
I’m a bit confused here. The product you’ve linked to looks to be for detecting flow within a hose, and the electronics and cabling don’t seem to be waterproof. I also couldn’t find any kind of depth rating on the Mouser page, or the Seeed Studio (the manufacturer) product page.
Assuming you’re somehow able to waterproof the electronics, I’m not sure how representative the flow measured through the fitting would be of general water speed, because the water going through is in an open system (depending on relative pressure, it can either go through the fitting, or around it), rather than the closed system that the product is designed for (all water must go through).
Beyond that, speed through the water can’t be used to directly provide a dead-reckoning position, because water can have currents and waves, which are ‘flow’ that’s independent of the vehicle motion. That may be possible to correct for somewhat with an accelerometer and gyroscope in an inertial measurement unit (IMU), but they tend to have quite noisy signals, which makes them poor candidates for longer term dead-reckoning.
Generally Doppler Velocity Logs (DVLs) and/or underwater (acoustic) GPS are used, although there are a few other alternatives.
Unfortunately most existing solutions are quite a bit more expensive and/or complex than your flowmeter idea.