You can't get your mage to GM in any school of magic until he is promoted to lich.He needs to be GM in Water to learn that invaluable spell Lloyd's Beacon. Also, it's extremely useful if he can cast Town Portal when hostiles are near, and he can only do that if he's GM in water.
GM Earth also give you access to what in theory is the most damaging spell in the game, Mass Distortion. I must say in all honesty, however, that I find Shrapmetal at contact range a lot more reliable and useful.
It's up to you - you're the one playing the game - but I wouldn't hesitate.
