Regen with figures in the order of 400-500 MP on expert water looks somewhat adequate and open for discussion.

Then it should be a level 3 spell (to reach where needed if you're just slightly short, like picking up a Pandora's box after clearing the way) and then the game will still boil down to searching for Dimension Door. Otherwise, one could at least run around normally even with a useless (currently) water book.

The numbers could be scaled from movement without penalty, restoring 40% of the maximum on expert; with full optimization/artifacts/logistics, that would be 1400 MP (option 10/20/30/40).