I have been to Hua Hin for the past few years. The best shrimp tom yum and other thai food i ever had is at the night market in front of 7-11 called Moo Seafood.

There are loads of sea food restaurants too by the sea if you walk past Hilton from Sofitel, if u would like it near the sea.

There are 2 night markets in Hua Hin and since you have loads of time there check them out and the various food stalls there to check out.