Mida de Sea is a little way out, and unless they have a shuttle, you will incur significant taxi expenses. Hua Hin has plenty of accomadation in a similar price range, and I'm sure you could find something more central. You can check out our accomodation listings/reviews here http://huahinforum.com.r24.asia/huahin/
I visited some friends at a beach front hotel next to the Mida de Sea a couple of months ago, and I wasn't impressed with the state of the beach there either.
I would definitely look closer. I'm sure somebody else will help with advice on high occupancy rooms near the centre.
As for the weather, yes, it is wet season, which usually means rain from 4pm until 6pm here on a bad day.