bass problem in a sailboat

It may limitation of either the speakers or pre-amp stage not delivering the frequency range or both. If all 4 speakers are in same area why not try getting new speakers? Two that deliver better low mid to high and two that deliver mostly low freq..
Just a thought.

I'll second theo. New speakers may make a world of difference.
180 watts is not a "ton" of power either. Consider more efficient speakers. There are some 2 ohm jobs out there that will sound a lot louder (produce more bass with less volume).
The drawback is that many speakers are that are rated at lower impedance are less efficient.
Finally, are the speakers in good enclosures? You may consider better enclosures or even speakers with baffles or enclosures attached. Look at Niles or Sonance for ideas with in-walls.

If you remount the speakers (tightly, waterproof, sealed well) in the bottom (hull?) of the boat so that the water itself acts as a cushion, this will provide just what you want. Not many people have the opportunity you have to use huge quantities of water as a suspension medium. Hemingway did this in the 40's

I'm still working on the problem of getting better bass from the system. As I said, I have a four channel head unit with four times 45 watts, 180 watts total. I'm currently using four 6" by 6" by 9" box speakers with it. The sound is generally good with the exception of very low bass notes. I'm missing really low notes that I hear on the home system. So far I've bought three things. I've bought a Sony woofer. I also bought a device which I thought would enable me to combine the bass from two channels of my head unit to run the one woofer. Fortunately a consultant from Crutchfield explained to me that receiver/"head units" aren't "bridgeable" and that the device is incompatible. He also showed me ( and sold me) a Profile HA700M, which is designed to combine two channels safely, amplify the signal some more, and run a woofer. However, I'm trying to keep this simple. I'm now thinking that I don't need ANY of what I've bought so far! I came across some dual voice coil subwoofers in Crutchfield's online catalog. I'm now thinking that I could simplify this whole installation and probably end up with better sound. I think I should eliminate two of the four full range box speakers. That would free up two 45 watt channels which could then run one dual voice coil subwoofer. The whole system would then consist of:
One head unit with four times 45 watts,
Two full range speakers, and
One dual voice coil woofer with two 70 hz low pass filters.

Thoughts, please?

With an open boat, the bass will simply dissipate into "thin air". Cars are enclosed and very small which enables the bass to resonate like mad.

First thing may be to find a spectrum analyzer to see what frequencies are missing. It may be quite surprising...

You may want to consider something like a "bass shaker". These little devices can be mounted either on you seat or in a boat, on the floor. You will need some sort of sub crossover which will send all of the signals below 60-80 to the bass shakers. Check out partsexpress.com for sub amps/crossovers. You may be able to run 2 of your channels to the amp/crossover and then to the speakers. The amp will divide the signal, amplify it, and send the bass to the shakers.