If the boat trip is important and you don't want to cancel, why don't you use that last little bit of adjustment to tighten it up and oil it too. There must be some adjustment if the last person to adjust it loosened it.
Then take it easy to the boat and when you disembark, find a motorbike garage to sort it out for you. Even if there is enough adjustment to tighten it up, I wouldn't ride at normal speeds or for any distance with that chain. If it comes off at any speed it could kill you, wreck your bike or at least ruin the holiday.
Stating the obvious, I would spend a few quid on your bike and have it serviced properly. It would be a good idea to have another biker show you how to look after it between services too. I don't mean to sound patronizing but you did ask for advise.
My Dad would have kicked my Donkey if I had gone to ride that.