men, the problem is that java don't have so many terrain level. (Someone know rpgmaker 2000? Almost like that XDD)
A way could be to create a new terrain type and set that Boat And People can walk on it (like in age of mpire = DryWater)
It dont use a level, but ther's another problem. The blue water is walkable, this mean that only the border line of the river is unwalkable. The devs should create another tile of borderline that allow to walk..is quite a not so fast work.
(If my supposition is correct)