Yes--I called DD and asked about their subs for my application, and they told me as long as they weren't exposed directly to copious amounts of water splashing on them, they should hold up fine. I was told sunlight was okay, as well, since many people have their subs exposed or "showcased" in their cars where direct sunlight will hit them regularly. I'm just concerned about placing them in an enclosure "out in the open". I keep my boat inside a covered slip when not in use, but there's always the chance of a shower on the lake, or kids romping all over the boat soaking wet.
Keep_hope_alive... you're not keeping my hope alive! I didn't even think about mildew... doggone it! However, one thing I was thinking about was computer packaging. I have several boxes from computers I've ordered in the past (I never throw packaging material away... go figure), and it has that rubbery, foam-like material in big blocks. I could even cut them into pieces that would fit snugly between the sub enclosure and panels. I think I'll try the pillow thing first, and if it works then move on to a more permanent solution.
If it gets to be too complex I'll just mount a 10-foot tall pole in the middle of the boat and place a down-firing sub on it. Bonus? Toss a disco ball on top!