[Bicycle Bob]

With a second float that is on a short string and drops a weight onto the valve. You probably also need a device to keep the main float from slowly lifting the valve. It should get almost submerged before a magnet or something lets go, and lets it jerk the valve open all the way.

P.S. This sort of action can also be accomplised by balancing a trough near the middle and filling it with water until it tips, lifting a weight on the short end, which is probably easier. I've got some ancient examples, but they are in books. I presume you can find stuff like that on-line.