Coast guard reguires you to have a towing endorsment even for assisted towing. Best idea is if you dont have experience or the equipment to tow another vessel dont! Assist in any way you can to help them but remember when you put the tow line on you become responsible for both boats and their crews and cargo.
I do not believe you are correct if you are offering a free no strings attached tow, which I have done with sail boats the same size as mine- boat was also about to hit a reef, so I would not worry about some piece of paper getting in the way. I think you are correct if I plan to charge or get some money. Also, the CG was aware of the fishing boat tow and did not talk about any kind of tow endorsment- they were just happy we helped and did not need to send the CG equipment out.