J-ROD,
A down-firing subwoofer has a very tactile feel to it. And, of course, you are benefiting from the effect of boundary loading. In order to preserve the sound quality attributes and maximum output in the upper bass you want to elevate the peak of the 10W7 surround at least 2.75" off the sole. That makes the box elevation even higher. That's a good number if the downfiring sub is venting 360 degrees. If the bass radiation is exiting in more of a narrow pattern then you would want to increase the elevation. The idea is to never reduce the woofer's radiating surface area at any point. Doing so will choke the output to a degree and alter the sound quality.
Is it done in vehicles? Sure, all the time. But bass doesn't propagate and dissipate in a vehicle like it does in an open boat, plus, in a boat you are far more dependent on the subwoofer for it's midbass contribution.
As for floor mounting, place a 0.75" depth stop on the drill bit and use a screw that penetrates the sole no more than 0.75". There can be a lot going on under the sole and none of it is good. Also, and this is especially important, do not screw into the sole within several inches of the outside hull. That area is normally coincidental with the hull making a sharp transition from vertical to horizontal. As the sole terminates to the perimeter and into the hull there is very little available depth.

David