Apperantly, everyone seemed to forget that you'll be installing this in a boat which means you should be looking at a sub that performs well in a vented enclosure.
The bass will be louder in an open area such as a boat and vented enclosures are much more efficient than sealed ones allowing you more dB off of 250w.
Also, the fact that you want to run @ 4ohms means your amp shouldn't sacrifice much of its damping meaning greater control of the sub at sub 30hz frequencies.
I would look into any sub that
a. Is designed for a ported/vented enclosure
b. Is made of weather resistant materials (no paper or foam)
To be honest, because of space requirements, I would seriously look into a BT1214 from Bazooka.