The best time to go to Thailand's beaches is Dec - March, apart from peak holiday times.
The worst time to go is probably May - Oct, especially on the Andaman [west] Coast [heavy rains].
The Gulf [east] Coast has a much less heavy rains than the west coast but longer, from June - Dec.
Thailand's best overall sightseeing time is the cool season, Nov- Feb. Worst is April - Sept, hot and sticky. March -May is the hot season. The rainy season is June - Oct.
The 'curse of the longtails' makes many previously desirable places a pain in the head now. These typical Thailand taxi boats are fitted with unsilenced truck engines, so put 2 or 3 together on water and it sounds like a 747 coming...and going....and coming....
Beaches that are only accessible by boat, or used as a longtail terminal have drone and diesel smell problems. Notably noxious are Krabi and the Phi Phi islands. Hey Mr Thai Environment Minister, put a noise suppressor on them!
This, the west [Andaman] coast has buckets of rain May-October.