Acts 27:31

εἶπεν ὁ Παῦλος τῷ ἐκατοντάρχῃ καὶ τοῖς στρατιώταις, Paul said to the centurion and to the soldiers. These would probably be able to stop the intended desertion better than the captain of the vessel. At all events they were strong enough in numbers to take the matter into their own hands, and cut the boat adrift. It seems too (from Acts 27:11) that the centurion had much to do with the direction of the ship. Probably he had chartered her for the conveyance of his prisoners and so had the right to be consulted on all that was done.

ἐὰν μὴ οὗτοι μείνωσιν ἐν τῷ πλοίῳ, except these abide in the ship. We see from this that every human effort was still to be made, although God had revealed to Paul that they should all be saved. If the sailors had left, the ignorance of the soldiers and other passengers would not have availed to save them at such a time. The skill of the sailors was to be exerted to carry out what God had promised.