I think the solution to this is for the guy who is doing the picnic to choose a better cleaner spot to take charters too.
we have over 20 miles of coastline and it would be prohibitive for anyone to pay to clean that as it is a DAILY errand and you clean it one day and the next it has garbage or seaweed again.
Most people who own property and live here have there beaches cleaned dailY i know I do but absentee owners have no way to clean there beaches until they come to live here and have to look at it daily then it gets cleaned pronto.
However the sloution is have the guide pick a nice beach that clean to do his bar-b-que
all the best