Small detail, but worth knowing before you need it: there's exactly one full-service bank branch on the entire island. Everything else banking-related either means an ATM or a drive off the key.
The one bank on the island
Truist has a branch at 5035 Ocean Boulevard, right in the Village. It's open weekdays starting at 9 a.m., closed Saturday and Sunday — normal bank hours, not extended for tourist season. If you need an actual teller on a weekend, you're not finding one on the island itself.
ATMs are easier to find than a full bank
Grocery stores, CVS, and convenience spots on the island generally have an ATM even without a bank attached. Fine for cash withdrawals, though you'll likely eat an out-of-network fee unless it's tied to your own bank.
The practical takeaway
If your bank isn't Truist and you're not planning to drive off the island, withdraw what cash you think you'll need before you settle in, rather than assuming you'll find a branch nearby when you need one. Most places take cards anyway, so this mostly matters if you specifically want cash on hand for parking, tips, or the odd cash-only spot.
Keep planning