You know, it's but a small distance between the embark and disembark points for those boats. Why not just have people pile into the boat, and animate it to ferry the players from point A to point B.
This makes it difficult for the invaders because shore-bound defenders could attack the boat and/or the players from the shore on the approach. Because, well, you can't really move around much in those tiny boats, and if you fall out, it's basically insta-drown. :P
This could implement the needed stopgap to give defenders the needed upper hand on the beaches. The invaders could click the boat to board, maybe cut it off at 16 players on board, and then the next group or two would board a different boat. (because 30+ players on one of those tiny rowboats would be a bit silly) The enemy could have a number of ranged defense (knights/conjs) and some marks/hunters on board as well, but they would still basically be sitting ducks until they land.
So, this would make it an option to invade the beach, but the beach would be fairly defensible. It would basically be range vs. range until the boat lands. The boat is on a set route that is self-steered (probably a captain or something piloting that the players do not control)
Optionally, defenders could try to defend by attacking the boat itself and try to sink it, thereby drowning all on board. The edges of the boat would be a "lip" like at the forts that you can't run over (and prevent accidental man overboard situations) but you could potentially jump over the edge by accident I suppose.
However, if nobody is there to defend the beach, then it would still be a pretty easy crossing.
It's just another thought, and probably easier to do than building a whole fort model on the beach, or adjusting algorithms as to when the boat would be available based on populations. It's really just an animation/script following a route/pathing with the players being placed on the boat after clicking it. :P
To beef up the idea, you could have a boat with a number of rowing positions. Players could elect to row to get the boat to land faster, but potentially taking away from people defending the boat itself.