Nice plan. Another question, how much do you think it will cost to build such a device? I mean, there's no RC-radio equipment involed so that'll probably spared a few hundred dollars or so. Another thing, perphas a PVC tube could be used for the electronics (instruments), and the wtaertight part of the capsule? When you're talking about going down to the bottom of a lake or pond, you're talking water pressure, so a PVC tube might be able to resist such pressure if the thickness of the tube walls is chosen correctly. You'd also have to make sure that the capsule lands vertically on its trigger.

How about using earth or soil as the solid ballast to be dumped? You could usually find some near the pond or lakeside, and it'll be pretty environment-friendly.