Thanks for the heads up bb. It's a pity because the CIMB website shows a ATM in central Hua Hin,, which would have been handy.
Still there is an Aeon ATM not too far away in the Market Village on Phetkaseam Road:
Market Village Hua Hin Branch #2
234/1 Market Village
Hua Hin Department
Room A 302, FL.3 rd
Phetkaseam Rd, Tambol
Hua Hin, Amber HuaHin, Prachuabkhirikhan
77110 3rd Floor next to Body Shape shop