Despite playing this game quite a bit, I experienced this for the first time yesterday
I was on a server with a buddy, we were stocking up for a skeleton ship mission
Then a galleon came over and asked for our boat. I was like what do you mean you want our boat!? :)
What this means is they ask one of your boat players to leave the game, and then ask you to invite one of their friends to your boat. Then your buddy leaves and leaves another open slot and effectively they then have 2 boats on that server. They then repeat for every boat they find, so after a few hours they own most of the boats on the server.
This way they effectively can get all the boats on that server, they alliance up and then they do athena / quests and quickly rank up. Effectively cheating the system.
Why is this an issue?
a) Well its effectively a form of cheating
b) By the time they got to us they effectively already had 4 boats with their players on, none of the boats they hijacked / owned already wanted to do skeleton ship missions, so for us continuing as a small solo sloop on that server, would have been pointless, so we wasted 30mins collecting gear! Yes we could have refused and just left but chances are they would attack us or if we left the new people coming into the same server would be presented with same scenario. I heard sometimes the people wanting to "buy your boat" may offer chests / skulls to make it worth your while.
My suggestion is that rare do some checks / trend analysis of logged in player names/tags, to see when the same 4+ players/gamer tags are constantly on the same server / alliances and warn those players / threaten to demote etc. Or look at this in combination with players ranking up too quickly via this method to highlight them, warn, ban / demote. Whats peoples views on this? Appreciate some people find it hard enough to get some time on the game, let alone co-ordinate some big server take over! But when it does happen leaves a sour taste.