I've had my bazi reading done and been told that my most favourable elements are water and fire, and that fire is my wealth element. Is that possible since water destroys fire? Wouldn't water destroy any wealth that fire creates?

If Fire is your wealth (zheng-cai or pian-cai), useful Ten Gods will not be fire and water at the same time. But fire and water can still be favorable at the same time.
You need to look at all the elements, their strength and etc at a BaZi and a particular Ta Yun/years.

For example when Fire is your wealth and you have wood and strong metal; in order to guard your wood and fire (in short your wealth/money)
you need water. Thus, you can say water and fire are favorable. This is just one of the many examples.

Cheers.