2-3 Towels: a single towel will not have enough time to dry between races.
Sandals or crocs for when you are out of the water
Beach chair or additional beach towel or mat that can keep you off the floor.
Parka or sweats/shorts/hoodie to stay warm between races.
Water / Gatorade / snacks to keep your energy up.
Battery for your phone.
Goggles (2-pair in case one breaks), Swim suit, and Swim Cap
Beach Chair - there is lots of waiting between events
Dress in layers. It can go from Jacket weather to sunny in a heartbeat. Hats are always good for some shade.
If you have an EZ up, its good to bring along. Just in case.
Fully charged phone. You'll want to take pictures or record your swimmer's race.