If you are getting very, very thirsty then you need to hydrate better before your dives.
Another common problem is dry mouth and throat.
To the OP:
Hydration is very important I wanted to re-iterate what Charlie said...
and remember... the air in a scuba tank is much drier than the air you breathe, depending on where you live this could be especially true.
When I dive my helmet with surface supplied air from a low pressure compressor (which is much moister than tank air) - I do 3 hour dives without feeling dry. 1.5 - 2 hours on Scuba, and i feel extremely dry and parched. There are some regulators that perform better than others with respect to this dry mouth, dry air thing. But I don't really know them off the top of my head.