How is it that they are renting? Most landlords and realty services wouldn't dream of renting to anyone under 25. They don't want the 18 year olds, but they don't want the 22 year old college kids either.
Are there college kids, or new high school graduates? I think it makes a difference if they have been on their own before.
I'm not sure I agree that 18 year olds can do whatever they want. If she's in your house and on your payroll for college or other support, she's not an independent adult, and you have some say. If she has been away at college for a year and living independently socially, if not financially, then it seems silly to restrict her. If she's just out of high school and has no experience in adult settings, this could be more problematic.
Is she paying for this on her own?
I think you have to be honest about what goes on at these beach weekends. They are alcohol-laden, with 18-24 year olds looking for a good time. I you are comfortable with the maturity of the group, and you know that they will have a designated driver (because they WILL obtain alcohol one way or another), and if you trust that someone in the group is sensible enough to call the police if their party is crashed by others, and if you think for sure that they won't let each other pass out from alcohol poisoning without someone calling an ambulance, then let her go.
If she is a naive type who says none of this could possibly happen, I'd say no to her. She's not ready. But if she's had exposure to these things, you think she and her friends have excellent heads on their shoulders, and they have not rented the property through dishonesty about who's in charge and their ages, then maybe you can let her go with confidence. You know your daughter better than anyone.