Here are just a couple of problems with an off-the-shelf automotive woofer box.
It's not necessarily the exposure to direct water that is a problem. It can easily be the air moisture content, especially when the boat has a little water in the bilge and is under the cover. A little heat from the sun and it is 'condensation city' in there.
They use water-soluable MDF. They typically use the absolute cheapest form of MDF from China. The lower seams are the first to fail and the enclosure will usually lose its airtight integrity within one season. BUT, the carpet covering conceals the failed seams for several more seasons.
Rarely do they use screws, air nails or staples for construction. Normally, the box has three pre-carpeted pieces only. The top, baffle face, bottom and back are one section of MDF with CNC'd V-cuts, and is then folded around the two end caps and glued....you guessed it....with a water soluable glue.
The enclosure integrity is not only important to the sound quality, but it is important to the longevity of the woofer.
I have never seen a car box in a boat that when removed several years later, even when it appeared perfect, that the dimensions were not increased.
Get with Odin at Earmark Marine. He has a variety of 10-inch and 12-inch fully marinized subwoofer enclosures ready to go. Plus, he can make sure that you get a woofer and enclosure that are properly matched. He's a really good sound engineer.