Not sure if you're talking about tax issues, or other issues?
For personal federal taxes, you can only write-off the incidental costs you incur for the charity cruise. E.g., if you provide a meal and drinks, you can write-off these expenses. But you can't write-off what it would cost to charter a similar boat for the day, nor even an amortized cost of your own boat.
We have donated a trip on our boat many times for private auctions at school event. As far as liability, I checked with our insurance company and they said there were no issues, since we were not being "hired" or payed for the donated trip. In other words, they said we'd be covered no differently than we'd be if we took some friends out for a sail.
Incidentally, two years in a row our donated boat trip netted the most of any offerings at the silent auction. We made sure to provide a real nice picnic meal (prepared by a caterer) with soft drinks, and were able to write-off these expenses.