Well, maybe just buy individual components for the bedding in “beach-y” colors. Shades of blue and some tan for sand. A tan crib skirt ,a medium blue crib sheet and different blankets in other shades of blue. Just plain fabric without prints, that way if you were to tire of the beach theme-the bedding doesn’t scream beach.
I think it would look elegant.
To accent I would use framed beach pictures, maybe a life preserver on the wall, a shadow box with shells, sea glass and bits of driftwood…that’s all I can think of for now.