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.
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.