Prior to Update 11, this skill was called "Amphibious", and increased your swimming speed instead of increasing your MaxMagicka. The swimming speed increase was moved to the innate Amphibian skill, and no longer requires the investment of skill points.
In Update 21, the MaxMagicka bonus was changed from a percentage to a flat value, and the resource restoration was reduced slightly. This update also increased the level requirements from 5, 15, and 30 to 25, 35, and 50.
This passive ability has been renamed to "Resourceful." Also, it no longer increases your swimming speed by 50%. It now increases your Max Magicka by 1/2/3%, and continues to restore 4/8/12% of your Max Health, Magicka, and Stamina whenever you drink a potion.
Gain 3% Max Magicka and restore 4620 Health, Magicka, and Stamina when you drink a potion → Increases your Max Magicka by 1000. Restore 4000 Health, Magicka, and Stamina when you drink a potion.