Mixed mono literally is just that. The left and right channels are mixed together so that they are equally present in all amplified channels. The easiest way to run an amp in mono is to sum the RCA inputs as shown with two y-cord adaptors. Then wire the speakers as normal.
I agree with David that there is no need for stereo on the tower, for the reasons he puts forth. I have to suggest that mixed mono for the in-boat speakers is not going to suite every ear or taste though.