In a similar situation I would have made an estimation of the approximate number of hours I needed to do the job from start to finish and charge them $-amount per hour. In my case I'm happy to do a dive with minimal risk of injury for $100/hr... so maybe $400 or so for that job from start to finish.
It's very important in jobs like this that you are clear about what you will NOT be doing or taking responsibility for. For example, where you attach the chain is the responsibility of the towing company. If you attach it where they asked and then they damage the vehicle in the process of dragging it out of the water then the costs of repairs etc. are for the towing company, not for you.... that kind of thing. It needs to be on paper and it needs to be signed.... yes, even for this kind of thing.
At my day job I've seen suppliers say yes to jobs that seemed really simple but were on the critical path of politically visible projects. In one case a supplier offered a job for 100k for some work that was on the critical path (of this he was not aware because he did not ask) and he had a 2 week delay in delivering some components and was held responsible for 2 weeks of delays of the critical path @ 200k per day. (ie. about a 3 million Euro claim for work that was offered for 100k). I work on big projects so these kinds of burn rates add up fast and contract managers don't play "T-ball" with inexperienced suppliers.
In that case it was a relatively small company with a certain specialism and they weren't used to that kind of scale.... It would have been a severe blow to the company to have to pay that. We mediated for them and averted a disaster but let me tell you that you do NOT want that to happen.
So before you say, "yes, I will attach a chain for $500", be clear that you will ONLY attach the chain and you will attach it EXACTLY where you are told to attach it and nothing more. That's the big message here.
R..