Air supplied masks are the best, for sure, but expensive (and your health is worth it), but I have used 3M masks while spraying Awlgrip and they do make filters for linear polyurethane paints. You might not be able to get them at West Marine, but pro paint shops will have them. Also, after reading your post again, do not spray Awlgrip or any urethane under the waterline as they will blister guaranteed if submersed long enough.