MDF (medium density fiberboard) is pressed paper fiber. The Sundown SA-10 woofer cone is pressed paper fiber. It will change its parameters when exposed to high humidity as in the marine environment.
The MTX woofer is a true high 'Q' 'infinite baffle' (IB) woofer that does not require a small and fabricated enclosure. But it still does need an enclosure in the form of complete front to rear acoustic isolation and a very large enclosure which can be created by a helm or seating console. In the case of a true IB 10-inch you would want at least 2.25 cu.ft. up to infinite.
The Polk Audio woofer is 'air suspension' and definitely requires a small fabricated enclosure. It is not intended for infinite baffle. However, you can use it infinite baffle with very limited power handling at very limited output levels. But, not recommended.
Two 10-inch subwoofers have more radiating surface area versus a single 12-inch. In pure output the advantage goes to two 10s if all other things are equal.
Air suspension has more linear excursion under control versus infinite baffle. In pure output the advantage goes to air suspension (small sealed enclosure) if all other things are equal.
So a case can be made for either.
If you are absolutely resolved about "No box", then you must go infinite baffle. A far better IB woofer would be the JL Audio MX 10-inch. These dual 10s would be best matched with an amplifier designed to load into a 2-ohm impedance and deliver an honest 400 watts. If you have more legit power then dual JL Audio M series is a better choice.
If you re-think using a box then I would have other recommendations.
You really cannot select the ideal woofer independent of a matched amplifier and matched enclosure.