You are acting as an agent for the charity.

If money changes hands, either to you or to your "contract employer,"
somebody could make a case that you were acting in a professional
capacity. It's at least a gray area.

I'd think that to be free of potential problems there would need to be
an arm's length relationship between
you and the charity- and no particular connection to the amount your
invited guests donated or even *whether* they donated to the charity.
Maybe a situation where you call your friend at the charity, invite
him/her for a boatride, and suggest they could bring along a half dozen
friends of their own if they wanted to. If the charity then, and
without your knowledge, dangles the invite in front of some potential
donors that is not your problem....exactly.
But somebody could still try to make it so.

A lot of the recreational boaters running out to get their
OUPV or 100-ton papers might do well to pause and consider that if
anything happens on their boat that means they will be defending
themselves in civil court, the complainant's attorney will make a big
deal in front of the jury about how a "licensed captain" should have
known better than to do whatever stupid thing caused the problem in the
first place. Heck of a price to pay for the ego zing of being "Captain"
so and so. If a boater doesn't earn his/her living running a boat, why
mess with the headache of being licensed?

The other thing that leaves a defendent additionally looking bad in
front of a civil jury would be a thorough examination of the "sea
service" claims made by the
recreational boater to sit for the license in the first place. It's
well known that a good many applicants lie, some of them a lot, and the
CG has no reliable method of vetting sea service claims. "If Captain
Zero lied about his sea service, why should we believe he's being
truthful now?"