A friend tried the bluetooth speaker thing, the problem was battery life, he was always running out of power when he wanted to use them.
In the end he just replaced the outdoor speakers with a decent (and not very expensive at all) set of marine speakers. Boat speakers are designed for poor conditions and for delivering bass (not just tinny transistor radio sound) with no real enclosure behind them.
If you want sound closer to where you are it would be really easy to install a couple of 1/4" jacks which would disconnect your outdoor speakers and send them to movable wired in speakers in case your "C" setting is as above not a third set of outputs.