Wet Sounds SDX-2 is a perfect option. But if adding in a woofer also, Id go with the SDX-6 for a pair tower speakers and up to 600W rms for a 4 ohm woofer.
Lots of good woofers. The best one is the one thats in the proper enclosure, powered correctly. From a construction point of view, steer clear of woofers with a paper based cone. Usually you will see terms like hybrid, laminated or cellulose in the description. Not ideal for a marine woofer. A rubber surround is better then foam. Aluminum basket is better then stamped steel. Enclosure magnet is better then exposed. Brand wise, im partial to Wet Sounds and Kicker.
Installation is not plug-n-play. Purchase through a knowledgeable and reputable source that can help before, during and after the sale. There are a number of details that can make or break a system. Having that assistance is important.