There's enough power in a bolt of lightening to do a lot of things, but we cannot harness it because it comes too fast and too hard.
While your proposal would allow the power to be distributed in time, in order to generate a signifigant amount of power, it would also involve a very large amount of force to be supported by the system -- the float would have to be extremely large, the forces involved in holding the float down while power is trickled through the mechanics very large as well.
In general, you'll find the most effective power systems harness low-level forces that are easy to build around -- a constant flow power that will not overwhelm the structure of a small device. So tidal tends to use horizontal flows where the water flow is effectively deflecting that energy into higher-speed, lower force form.
Tidal is a promising avenue I would say. My personal favorite is wave power, but I think we need to attack this on all fronts with every form of generation we can muster.
(EDIT: Oh and by the way, off topic, but you might like this little beast here:
http://en.wikipedia.org/wiki/Underwater_gliders)