I want edit my post just a little after double checking the specs on the ZX250.2. At 4 ohm, such as driving 1 speaker per chnl, its output is 85W rms. That is by no means, grossly under powering those Kicker components. Although they would run comfortable at 120W, I do not believe upgrading the amp at this point, would yield the results you are looking for. I agree with David, if you are ready to make the investment, then go with a single pair of larger speakers. Picking up a 2nd zx250.2 would out you spot on for most any 8" tower speaker out there.

Zone wise, I feel that the in-boat and tower need to be different zones and designed to achieve the goals that the owner wants for each. IMO, the sub is part of the in-boat zone and in most installs, stays that way. 9 of 10 customers with towers, do not want a 2K watt in-boat system to compete, keep up or be in balance with their 2K watt tower setup.

For the tow-sport enthusiasts, most are after a setup that allows them to tone down the in-boat volume when the rider is in tow. This allows the rider to enjoy the tunes without the passengers being subjected to the in-boat volume at the same time. So in those cases, an in-boat system that will keep pace with the HO tower system is not practical. There is the exception, though. There are those that funds are not a concern when designing the system. In most cases, they spend more time at the party cove. Their days on the water are less about tow sports and not too family friendly. So for them, having a 2500 watt sub projecting 6 boats over to the beer pong table is what their goals are.