Jaguarjoe gave a pretty good answer, but to get more & better answers we need some clarification. Do you want to measure how far TO the bottom from your current position? Or.... Do you want to determine how deep you are under the surface? What max depth do you need to measure to, and how much resolution do you need?
If you don't want to cobble together your own (and probably low power=short range) ultrasonic depth finder, you should be able to wire up to a small boat depth / fishfinder transducer. They have a few hundred (or even low-thousands) Watt pulsed output to achieve multi-hundred foot ranges. Pulses are short, total average current draw isn't too bad.