Its a problem these days.........
Sadly its the same problem with any formalised / organised cruise or event these days. If you and Mr B are organising an event, folk are paying for it even if the proceeds are going to charity then there is a risk, however slight that if someone was injured either on the event or by a boat in the event, then you as the organisers could be sued. The ott example we used to discuss endlessly at BIBOA committee meetings was a RIB hitting a boatload of stockbrokers each earning £250k p.a. who then are injured and off work for a year. £1 mill claim for lost earnings here we come.
IMHO there is a world of difference, legally and liability wise in a bunch of people on rib.net saying "lets meet up on sat on the solent" and someone saying "i am organising an event, pay me x to attend (although for a good cause)". As event organisers you have both a duty of care and a risk of liability if something goes wrong.
You might be able to get liability insurance but suspect costs would make event prohibitive to run. BIBOA's liability insurance for officers and committee (& by extension cruise organisers) is a signficant part of the clubs total spend. Liability was also the reason for changing from a club to a company limited by liability.
OK so having said all that what can you do? Here's some thoughts:
1) Find out what DW do for their poker runs.
2) Goahead anyway and rely on people being sensible and nothing untoward happening.
3) Turn it into an informal cruise with mebbe some sort of charity auction/raffle/quiz at a pub to do the fund raising bit
4) Organise it under the auspices (& liability insurance protection) of a club such as BIBOA (Who I am sure would be happy for this to happen but would insist on skippers being members) or mebbe the Royal Southern YC (See Martin's thread about predicted log)
I agree that its a complete arse & a consequence of the compensation culture/nanny state but you'd be daft not to think seriously about it.
All IMHO/FWIW and the value of my advice might go up and down!