The towed boat doesn’t accelerate perpendicular to the stream it is being pulled along. In other words: $a_x=0$. Furthermore, maybe the forwards-pull just balances out the stream so there is no forwards acceleration either (just a constant speed): $a_y=0$.
- ‘Like “zero vector”, do we have a “one vector”, “two vector” etc.?’ (web page, forum conversation), Steeven Hegelund Spangsdorf, Mathematics Stack Exchange, 2019, math.stackexchange.com/questions/3386960/like-zero-vector-do-we-have-a-one-vector-two-vector-etc (accessed Oct. 9th, 2019)