The whalesharks do not have a season (anymore) in Thailand; they are unfrequently seen year-round on most divesites. Before 1999 it was not unusual for a live-aboard crew to see up to 40 whalesharks in one-single season (october thru may) with peaks of sightings in the months March/April and May (especially at Richelieu Rock).
Nowadays we count ourselves lucky if we see 2 or 3 animals a season! Times surely have changed...
Stop looking for whalesharks during your dive and rather spend your energy looking for critters that still CAN be found on Thai reefs. Ornate ghostpipe fishes, tigertail seahorses, painted angler fishes, red bar anthias, harlequin shrimps, reef stone fish etc.
If a whaleshark DOES show up you will notice it, I promise!