The rule as I understand it is that you have to come to an agreement before the assistance is rendered.
Practically I'm not sure how you would get a chance to read the contract and sign it but that is the law.
I believe a verbal agreement is binding but of course harder to prove.
Even if you have a sea-tow membership and and you call them and it is a hard grounding and requires extra equipment and boats you are probably looking at salvage. The fine print is interesting.