Becoming dehydrated as a result of living in a hot climate does result in an increase in their blood glucose level principally due to the reduce quantity of the blood that now flows throughout their body.
As you rightly say, the answer is to consume adequate fluids, ideally pure water to compensate for the loss.
Similarly when a diabetic individual is ill, their body in fighting their illness can also give rise to an increase in their body temperature and hence cause a milder form of de-hydration, and as with living in a hot tropical climate, it also is important when the individual is ill to consume adequate fluids.