Hidden Power isn't dependant on personality, it depends on the IVs your Pokemon has. For Hidden Power [Water] you need-
E O O E O E
O O O E O E
E E E E O O
O E E E O O
This means that the IVs in each respective stat must be odd or even as shown above. The first on is HP, the second one is Attack, the third Defense, and the rest are Sp. Atk., Sp. Def. and Speed in the respective order. So your IVs must be like the combination above or else you won't get Hidden Power [Water]. I suppose you are trying to cover Ground-types, but I reccomend Hidden Power [Ice].
Ice O O O O O E
Ice E E E O O O
Ice O E E O O O
Ice E O E O O O
You should aim for the first one. It gives you the ability to have 5 perfect IVs(31 is an odd number, I guess you know that), and the speed IVs don't matter anyway. It also covers Grass-types(Electric is not very effective on Grass). As for Nature, I would go for a Modest Nature because it boosts your Sp. Atk. at the the cost of your Attack stat, which is useless on Magnezone.
Source
^The Hidden Power Types and the IVs required.
Hope this helps!