So... now that there are boats... there are MANY ways to get into towns ILLEGALLY (though same could be done by walking in shallow water...), so my idea is ability to make gates between walls on each sides of river... which could be opened while on boat, or on shallow water.
Requipments to do gates i can think of are being in villages area, have atleast 1 cornerpost on each side of river, and make the requipments for materials grow the longer the gate is.
the basic requipment to make a 1 tile wide gate (not really possible in game, but whatever) would be 50 boards, 80 logs and atleast 1 common metal bar (boards to keep the gate over water, as a makeshift land) and double for 2 tile wide, and triple for 3 wide