You and I agree to go hear a concert together. We agree to drive in one car. We park in the parking lot, get out, and begin to walk to the door. We are two buddies who have agreed to go hear a concert together.
Suddenly a guy with a ballbat jumps up from between two parked cars and commences to whack you with the bat while loudly demanding you turn over your shoes. You look at me and say "Dude, I could use some help here"... I flip you off and run away.
I am certainly violating a moral duty to help my friend, and socially I'm a big coward and worthless human being.
But our agreeing to go see a concert together did not constitute a legally-enforceable contract that holds me subsequently liable for failing to come to your aid. I may have taken on various moral and societal responsibilities by agreeing to go together with you to the concert - but there is no valid, enforcible (in a court of law) legal contract between us.
To the best of my knowledge - and if you can provide a legal citation I will happily declare myself utterly wrong - it is the same with agreeing to be dive buddies.
Such an agreement does not constitute a valid, legally binding (in a court of law) contract between us that requires either of us to render aid to the other or be prosecuted for failing to live up to the terms of the contract. We are not held to any higher standard of duty or care, as might be the case if we were, say, police officers or medical professionals. For one thing, the court would need to examine the precise terms to which we contractually obligated each other. And my understanding of your obligations may be different than your understanding or anticipation of mine.
Again, I may be mistaken - but TTBOMK dive training traditionally identifies moral or professional ethical obligations to one another, but not legally valid and binding contracts between one another.
Doc