I like the "M&M" acronym
Nowadays the various pepper sprays available are quite efficient and work wonders on just about everybody. But use of any aerosol on your boat will guarantee that you will get some as well. If you, or anyone on the boat that you cherish, has any asthmatic conditions including hay-fever type reactions then OC/Mace/Pepper Spray might be as bad for you as for your opponent. Anyone with asthma should avoid this type of self defense and when I sell this for self-defense purposes what I usually do (when possible on windstill or calm days) is to go outside, spray a small spurt in the air, wait 5-10 seconds and then I and the customer walk through the remains. This is about the amount you will get from backsplatter if you use it inside and immediately get out of the area. If you have to use it against the wind or need to stay in the enclosed space you will get even more. Anyone showing strong adverse reactions should best stay away. Note that these materials all have an oily base and WILL adhere to inside surfaces; it is easily washed off wooden surfaces but you might have to launder your cushions.
Aerosols are still the best LTL defense out there! Fire extinguishers can also be used to good effect (although they have even more cleanup), plus you can always chuck the empty canister as well.
Quite often, even during the day, the effect of a powerful flashlight is overlooked. Particularly a lithium-battery powered flashlight like those made by Surefire
; I have one of these in the stateroom both for normal use and for the shock effect.
I also found that a strategically placed, well-sized and sharpened dive knife is a great help in finding deep sleep in troubled places. The positioning should be somewhere where it is directly grippable by the strong hand when leaving the bedroom or cabin. That, and a piece of string tied across the companionway or ladderway and attached to a small bell is an alarm that never runs out of batteries.
Firearms on board really are a tough subject. If one complies with the laws of most countries one visits they are a nuisance at best since you never have them aboard. If one doesn't comply with the firearm laws one becomes a criminal and potential cellmate to "Bubba" (plus risks losing the boat). Plus a handgun round doesn't compare well to a 7.62x39mm round.