What does that mean? He used the boat because it was Necessary, so he has to pay?
In any case, "necessity" as a term of law is an affirmative defense.
Say I tie my boat to your dock (trespassing) seeking safety from an approaching storm (I have no other reasonable alternative). During the course of the storm, my boat damages your dock. You may sue me for damages, but I can use "necessity" as a defense. It's then up to the jury.