Sure thing. It's the NPC's job to do it, after all.
Although, now that I think about it, the raft probably has a speed of 15' without anyone having to make any checks. Usually 5e doesn't make people make checks to move (even on a vehicle) unless they want to do something fancy (or if there's some kind of obstacle).
So I won't be mean enough to make you roll just to move. Just tell your raft's NPC where you want to go and he'll take you there. He can even "dash" your raft 30' by using his action.
I *will* rule, however, that whatever movement the raft goes while you're on it comes out of each passenger's own movement. (For example, if the raft goes 20' you only have 10' of movement left if your speed is 30).
In my mind it would be weird to have someone *gain* movement in a round by riding a slow-moving object.