I think a bike cover is a great idea anytime you're not on your bike. Beach sand gets everywhere, even when you take precautions, especially if there's any kind of wind at all. Fine particles to course grains, you and your bike will be coated, and if the beach is at the ocean, then plan to deal with the salt, too. By the end of the day, you'll likely feel a little crusty, even if it's not especially windy.
You might consider some leave-in conditioner for your hair, too, to help protect it from the salty air.
Have a great trip!
Roxy
Getting in touch with my inner try-athlete.