I would almost bet that it has something to do with the movement changes of boats !!!!
lol okay seriously What is probably going on is they have some construct the program goes in to when you move your boat to update its position in the world, well instead of it doing it once for the whole boat, they probably got it going though the construct for every tile the boat takes up so every time you move your boat if your boat is 10 times its got to make 10 updates to some database as where your current position is in the world, Now times that by 100-300 players constantly moving around at sea and you end up with the database having to deal with 3000 updates a second instead of 300. Just my guess /shrug