Dont build a 12x12x12 box. remember to calc based on internal volume. also use 3/4 " wood. not 1/2" make sure to glue and screw the box and caulk it too.

i have a box with 2 alpine subs they are 10 inch you can have for 125 plus shipping i paided over 500

The simple solution is to just use the engine compartment as an infinite baffle. Unless you're really trying to make super low frequency bass, an infinite baffle enclosure is really efficient and doesn't take up any space on the boat or add unessesary mass. And no, the engine compartment does not have to be sealed for it to work right, the distance from the front of the sub around to the back just needs to be longer than the wavelength of the lowest frequency you are trying to reproduce. Not really that hard given most music you're going to listen to on a boat doesn't really need super low frequecies (20hz), just a lot of 40hz and above.

Exactly A sealed box will be different then a ported box with the same giving speaker, sizes will change on box area based on the make model sub ya have..

John jr