Clayman,
If the yacht is climate controlled then indoor APs are obviously the way to go. Fiberglass and/or wood are conventional materials and won't impede signal propagation much more than you'd expect in a typical indoor environment. Much easier to design for than steel bulkheads.
If no climate control, then indoor APs are still an option but you have to make a call about whether to put them in an enclosure. Without knowing the details of the yacht it's hard to guide you. If you can provide some more details I can be more specific.
the AP175 thin AP and MSR2000 autonomous AP are IP66 rated and corrosion resistant. But the enhanced cost of these units might make them a challenge on a smaller boat. Also, you'll need external antennas for these models.
External antennas can be an advantage - on a larger boat where aesthetics are critical, a patch/panel antenna connected to a remote AP may be most attractive and/or least intrusive.