Pick Your Destination Wisely
Some destinations are better rental spots than others. Many Jersey Shore beach towns, for instance, are heavy on rental homes and lighter on hotel rooms. Same goes for places like Martha's Vineyard, Massachusetts; Tybee Island, Georgia; or Gulf Shores, Alabama; these are great destinations in which to rent.
If you're not too familiar with your prospective beach destination, investigate the availability of different types of properties: rentals versus hotels or B&Bs. A rental might not be your best option in your beach town of choice. Compare prices and amenities. And go from there.
Summer is low season for some beach destinations, such as the Caribbean and Costa Rica; these could be a great option for deals if you're OK with paying for flight costs and your passport is up to date. (Summer coincides with hurricane season in the Caribbean, though, so watch out for stormy weather. Consider purchasing travel insurance.)