Cruising Kitty etiquette
Your desire to help with your hosts'' kitty is very generous. The answer to your question should take a number of things into consideration: the financial situation of the owners, the lenghts of time you will be their guest, the additional expenses they will incur because you are aboard.
My wife and I had numerous guests, but never expected them to contribute - they were our guests. However, most of them insisted on paying for dinner ashore or for the provisioning expenses for the time that they are aboard or for the cost of excursions which we would not have taken if they were not aboard. I would simply say "I hope you are not offended but we insist on paying for.........." If you will be spending more than a night or two aboard, I hope that you know them extremely well and you should be able to determine where the line is.