I would first find out from the church how long you will have to decorate and take down any decor. We only did bows on every other pew for my daughter’s wedding because we had limited times afterwards. You could do something similar and/or use large lanterns with candles, sand and shells or starfish. Keep in mind though you’ll need a coordinator or someone to take it down afterwards.
Lanterns would be nice. I would also ask the church about any restrictions. The church I got married in didn't allow candles except the unity candle, petals, aisle runner and some other things I can't remember now.
Have you picked your florals and greenery yet? Using tropical plants might be a good way to bring the beach theme into the church. Bird of paradise, orchids, anthurium, hibiscus, and split-leaf philodendron could all look absolutely beautiful.