Hmm. I didn't think much about this riddle when I threw it together, but I think this final edit makes it spot on. Hint: what exactly IS rain? (This is going to be one of those head-slappers)
A full moon sits now o’head,
Waves and rocks at hand,
Eighteen score, no less, no more,
The cold and careless band.
Plastic, tree, OR cloth ‘tis me,
I hold the thing you covet;
So long as rain from deck abstain,
My purpose won above it.