Can I bring in a refillable water bottle?

Absolutely!  The vessel MUST be empty when you enter the event.  This includes Camelbacks.  No exceptions.  There are igloo cooler water stations located throughout the event space that you can use to fill up your vessels.  A tip: let the water in the cooler run a bit, then it will get cold.  Remember, we ARE on a beach with hopefully a lot of sun!