Ideas and suggestions.

For MP regeneration (level 5), I think it would be better to give a fixed number instead of a percentage, so that regeneration doesn't depend on logistics. Let the numbers be like this:
* +250 without the Water school
* +500 basic Water
* +750 advanced
* +1000 expert
The mana cost is similar to other control spells - 20 mana without the school. + a limit of 2-3 times a day per hero.

2-3 times a day for 1000 movement points? This will be skill #1 in Heroes, the hero will be chosen based on the principle of "leveled up Water - go to the back ranks," just like diplomacy on Anarchy. What logistics... water mages!!! The goal is to make Water the best school in Fortress.

Reasonable numbers would be something like 200-400-600 and 1 time per day.

Although it will be a bit boring, the day will start with casting this spell, and then everything will be as usual. There isn't much room for the skill...