I've not flown seaplanes, so take this FWIW......

The effectiveness of the retrival system depends on the situation your plane is in. e.g. If you do a good landing and are upright but stalled in the middle of the water then a couple of systems would do the trick e.g. an electric motor and prop installed in the floats or hull, or a tug boat to push or tow the plane.

But what if you've flipped the plane over? Driving it wouldn't work, and attempting to tow it may just drive it undrwater.

If you're on an enclosed lake there'll be no current and the water should be fairly calm. In which case you could just get a cheap 2-3 man inflatable like the ones you buy at the beach. Get yourself a 12v inflator and a paddle, and the whole kit 'n kaboodle is fairly compact until you actually need to inflate and use it....