Answer
A tarantula sitting over or beside its water dish may be drinking, resting there or simply using that part of the enclosure. One visit to the dish does not prove dehydration.
Look at the whole spider
Check abdomen condition, posture, coordination and recent moult history. A healthy spider can spend time around the dish without any problem.
Check the substrate
If a moisture-dependent species is repeatedly clustering around the only damp area, check whether the rest of the substrate is drying too far.
Do not remove the dish
Keep permanent access to clean water. If the spider buries or webs over it, clear and refill it.
When dehydration is more likely
A markedly shrunken abdomen, weakness or poor coordination alongside lack of water is more concerning than location alone.