A.T. Robertson Commentary Acts 27:31

"Paul said to the centurion and to the soldiers, Except these abide in the ship, ye cannot be saved." — Acts 27:31 (ASV)

Except these abide in the ship (Εαν μη ουτο μεινωσιν εν τω πλοιω). Condition of the third class (undetermined, but with hope, etc.). Paul has no hesitancy in saying this in spite of his strong language in verse 24 about God's promise. He has no notion of lying supinely down and leaving God to do it all. Without the sailors the ship could not be properly beached.