Since there was some interest, I'll give a quick update. I ended up getting one of these: http://www.ebay.com/itm/Waterproof-Ultr ... 0721354139
and putting it in a waterproof housing. Preliminary results in the swimming pool show that it can measure from 4 to 12 feet pretty accurately. In theory it should work out to about 35 feet, but I won't get around to testing that until the end of this week/early next week.
During tests in air, I noticed that the sensor wasn't getting good returns off of a person at 6 or more feet away. It did work well with large-ish flat objects, however.
This opens up possibilities for having cheap AUVs be able to navigate around a pool (my project), or having rovs that can detect (or possibly hold) their distance from the bottom (with a minimum range reading of 4 feet). It could also help you avoid running into large (flat?) objects in murky water.
Hope someone is able to use this information to make something cool.