Hello @Carolyn139,
I would contact Airbnb and request that they cancel the reservation penalty free for you.
Tell them that you're uncomfortable with the reservation.
You may have to emphasise other points other than the fact that they indicated that there will be a party as you don't have "not suitable for parties / events" in your house rules. You could stress that they indicated that there will be 18 people on board when the max is 8. Is there a local legal / permit limit, e.g. lifejackets, on the number of people that can be on board?
Some points to consider.
If you don't want parties put this in your house rules. They may have booked it as you currently don't say "not uitable for parties/ events".
Airbnb contact deails can be found here Contact-Airbnb-A-Community-Help-Guide-UPDATED
Steve.