Music straight from the cell phone won't happen. You need some processing. That being said a $100 miniDSP with a volume knob would kick ass for that. Other option is to make sure that your pro audio tops can cross themselves and buy a sub amp that also has a crossover. Then you can split and balance indepently. You may find that the amplifiers however do not like the input from your cell phone since it will be very low. If you have hiss you'll need a line driver as well.
In my boat I have a ported Xcon 15" with 1kW on tap. It makes the boat hump, but if I were doing what you are I'd take it up a step further and at least do an 18". I'd probably build the sub box into the cab/cabin or at least be flush with it and keep it semi-permanently on board. The highs I'd put on pillars coming out the front of the boat up by the points of the pontoons aiming backwards. Something with a rather directional horn instead of a widespread will also be better.
The best modular approach is to do the sub/amp/passive volume knob/and some used pro-audio monitors. If you don't love the monitors your first upgrade will be to sell those and get bigger ones. They really will not work at all without a sub. Believe me. I know how to get girls to remove their swimsuits on a boat and load isn't the goal, you NEED bass.
Of course, a linedriver may be the first update if it hisses hot and heavy.
**No matter what you do, I would highly suggest an analog or any volume control in between your phone and the amplifiers. Other people WILL screw with your play lists and volume and without the extra protection in between you will end up with blown shit.
In my boat I have three zones (wakeboard boat). Z1 is the in boats. Z2 are the tower speakers. Z3 is the sub. Each have their own volume control. I also use a 4th volume knob for overall. I have no headunit either as I use an Android tablet connected to a 1Tb hard drive of music. My system was similar in price to yours, but I'm building my speakers for integration which makes it a bit more cost effective. You need to get there by buying some used monitors. Start shopping