Well, they came here somehow and we didn't see any boat. If they don't know how to operate it then we're safe. If they expect payment we're safe. If they don't want bigger group looking for them in the city, we're safe. Call it calculated risk
We can always travel around the lake and be pissed at them when we drag our sorry carcasses back.
I don't object to you objecting in-game I really don't expect them to steal barely seaworthy vessel or even to accept guarding it.