The trouble with the ones that are in the middle of nowhere is that they are expensive to get to and or expensive to stay at because the cost of bringing the services to you are costly.
Check out www.asiarooms.com for Krabi, Hua Hin or Pattaya.
Krabi, because of the layout of the city, there is not any Spa hotels on the beach. Check out the maps for your judgment.
Hua Hin, there are only 2 that says they have spas, Marriot and Hilton. They are very nice for the price. But, not a deal.
Pattaya, has a few:
Aisawan Resort and Spa, north Pattaya near the city of Naklua
Long Beach Garden and Spa Hotel, in Naklua reasonable price at USD 68 per night
And again a Marriot
The best bet is the Long Beach Garden and Spa Hotel. It's away from the main part of Pattaya on the north side and no where near the walking street in South Pattaya. And the price is not bad. A bit high but for a 4 star hotel it is very reasonable I think.