Every seat on every boat is numbered. They are given out at the jetty rather than chosen online, in the order the bookings came in, and we do it that way for one reason: a boat where families are split up is a boat with a crying child in it for fifty minutes.
So anybody travelling with a small child is seated together before anything else is allocated, then groups, then everybody else in booking order.
If you want the front, which is drier, or the back, which moves less, ask at the counter. We will do it if we can and say so if we cannot.