Water!!! Because without water, you can't go anywhere!
Summoning a boat + SH (presumably referring to a specific skill or ability) instantly solves the problem if there's even a single water tile!!!
Secondly, almost all spells become useless near water... or simply lose their effectiveness.
Of course, Mass Blessing and Prayer are good spells, but the rest...
As I said before, I think the best school is Earth.
: )