Don, here is my suggestion...
Buy some nice moisture-resistant box speakers with metal bracket mounts and get a pair of those nice white rail clamps (which are movable and removable at any time by using a single knob - made by SeaFit, I think, and available from WM). Drill mounting holes in your metal speaker brackets to match the holes in the clamp's plate, and mount them to the speaker brackets with steel bolts and aircraft nuts. Get two Perko watertight deck connections and mount them in the stern area above the cockpit near to where your speakers will go. Wire the female receptacle of each connector with #14-16 double wire to your stereo/CD speaker leads. Now wire a length of double wire to the male plug of each connector, and attach the other end to each speaker using "liquid electrical tape" over the speaker terminals. (Be shure to match the +/- polarity by wire color). Now the speakers can be simply unplugged during wet weather, washdowm, and for theft protection at any time, and if you leave enough slack in the speaker wire, they can be moved anywhere on the rails vertically or horizontally to suit. (If you mount similar receptacles in the interior, they can also be moved inside at any time without the need to buy additional speakers!)I think this was harder to explain than actually do...