In the instant case, is there any evidence who did it wrong? What went wrong? Does the author say specifically whose fault it was? So we can not assume.
Let's say if you are the owner and the boat came back damaged and nobody would admit who did it wrong, what would you do? Most courts will treat the case as joint and several liabilities and the defendant has the burden to prove that he did not do it. Even if there is only one defendant named, he could in turn sue other defendants for contribution.
This is tort not contract so that only one guy signed the agreement does not release liability of others.