proper subwoofer enclosure

Sealed or ported, I do not know. But I can tell you to mount them as far away from the listening area as possible. Front of the cabin firing toward the back of the boat will not disappoint you.

now with a sealed box am i better off to get an enclosure that is perfectly square or rectangular or one that is slanted either on the front or back like a hatchback box.  the reason i ask is because i am under the understanding that with a slanted box you will eliminate any possibility of standing sound waves or sound wave cancelling

The shape of the enclosure is not important when dealing with long subfrequency waves.  Now I would always first try to design a box a little odd-shaped (like "golden ratio" 0.62 : 1.0 : 1.62), but build it so that it fits your application best.  The golden ratio would be of much more importance with a mid or bass design.Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

If i was to go with a sealed enclosure I would either go with an mdf enclosure at 1 cu. ft. per sub (that is what manufacturer recommends) or I was thinking of possibly going with fiberglass enclosure that is also sealed with 2.25 cu. ft. total box volume but it is also a mono chamber box.  does anyone have any suggestions. please keep in mind this is for a four winns boat.  thanks

I think i would be going sealed. It is for a boat. And there will be water that can get on it. Ported you have the chance of water getting inside the enclosure. MDF I think would be fine. It not like it is going to get soaked all the time. It shouldn't be placed where it would, but I would put a water resistant material like rhino lining or something similar. As for the chambers I like separate cambers. If something happens to one sub, the other will still be in a recommended size enclosure. The only way i run more then one sub in a single chamber is when restricted by space. You can get away with a slightly smaller enclosure if they are in the same enclosure.