I don't think as a general line of defence , jumping in the water is a good argument. Doesnt work if menaced down below and/or assumes you can reach an exit.
Furthermore assumes water is warm. Cold water shock could kill or disable you very rapidly.
Trial streams etc could carry you away.
If you are challenged , the mere act of turning to mount the side decks and jump could be misinterpreted and lead to an attack possible aimed at your back or outside your vision. Finding yourself in the water , reliant on keeping yourself afloat ,with an injury is worst then remaining on the boat
In extremis, it might be an option, especially if the opportunity presented itself , so as to be acted upon in relative safety
, it could be used . As a " strategy" I don't think it merits.
The vast majority of boardings have simple robbery in mind. The key here is to not directly or inadvertently surprise such people , surprised people react. IF you surprise them , you need to be confident you will win the outcome , Let them take what they want.
Active physical self defence , needs to have a particular mindset and ability because once committed there no going back. In most people such reactions only generally happen when it become apparent that ones life is in danger
, and then it's often too late
In my view , either you have a deployable firearm , primarily used to deter ( no one what's to shoot anyone ) And you get the opportunity to retrieve it , load it and use it, or you really only have a passive play , attempting to convince the assailant you are not a threat.
Thankfully most petty criminals do not want contact with property owners , preferring to steal while unchallenged, the rare other types are more difficult to deal with.
The best self defence advice , is simply to not put yourself in a place or position , where such violence is prevalent or where your senses warn you something is wrong. Deploy alerting equipment
if you can , as forewarning is the greatest advantage , use passive defensive systems , like good locks , physical barriers to entry etc. do not " flash the cash"
All this will get you through 99% of places the other 1% ,sail somewhere else. After all its a boat.