Brian,
If you double your 'amplified' sub you will pick up another 6 dB of output. 10 dB would be percieved as twice the volume so you would be a healthy increment of that.
Two subwoofers in separate locations will not sum as efficiently as a symmetrical and compound arrangement in a single location.
If you want to be "that guy" that when tied up can dominate several rows to the side and severals rows behind or you want the bass to extend out of the cockpit and beyond the transom to support the tower speakers at distance then you need much more subwoofer and subwoofer power.
However, you can overdrive the inboat coaxials with too much bass if the in-boat listening is your primary focus. You will find that when listening to a subwoofer all by itself it will sound soggy and muddy with little to no bass discrimination (everyone should try this once). Add the coaxials and you will discover the pitch accuracy and tonal definition return to the bass. For musical sounding bass you need a precise balance between the fundamentals and upper harmonics. Pushing the subwoofer makes it sound remote or detached. It lessens the musical plausibility.
It depends on your personal 'taste for bass', usage and music. Is it for dance, Hip Hop, a party boat, getting noticed and the like? Then you absolutely need more bass.
Even if your taste runs a little more conservative I think clean bass is never hearing the limitations of your subwoofer or the power driving it. So personally, I always like extra in reserve because I have zero tolerance for over-driving equipment and the tonal aberrations that it introduces.
Because I am so picky about the bass accuracy some of the best and most integrated systems I've heard used a 10-inch subwoofer or a larger subwoofer with larger 7.7-inch inboat coaxials.
You just cannot impose your personal preferences on others. You have to be tolerant of others expectations. Its truly different sizes for different folks.

David
Earmark Marine