What a nice idea!!

You're right about it not being much radio controlled but a device like this could grow up the envy of any sub-fanatic! You can perhaps use a timer to calculate the time it takes to reach the bottom (and going up!) couple that by the speed it takes for the boat to reach the bottom and you'll probably get an accurate distance calculation. I wouldn't call this much of a "boat" but perhaps rather a "capsule". This could be really useful for any sub modeler who wants to run their boat in a pond or a lake, by dropping this to the lake's bottom, you could get the approximate depth of the lake in advance, before running your boat just in case something happens.

Be sure to let us know your progess if you plan on continuing on on this idea.

P.S: I don't quite understand your last drawing, is it the trigger that will activate the ballast release mechanism?

Looks very promising!