As an aside, does anyone know if Hydrosophist bonus affects this bonus?
Yes. It works like this:
The amount that Necromancer heals is equal to X+D(0.03*X) rounded up, where X is the amount of points in Necromancer. So if you deal 1 damage and have 1 point in Necromancy, you will heal ceil(1+0.03) = 2 health. [ceil=ceiling; rounding up the number]
This is analogous for the Hydrosophist bonus (let's use Y for the amount of points: Y+H(0.03*Y), where H is the base heal).
So if you have 1 Hydrosophist as well, the healing increase is equal to ceil(1+2(0.03)) = 2 health.
So you heal 4 health.
Example with 70 damage, 5 points in Necro and Hydro:
ceil(5+70(0.15)) = 16 health from Necro;
ceil(5+16(0.15)) = 8 health from Hydro.
So you heal 24 health.
In addition to every instance of direct damage to any character (including party members), status effects caused by you will also heal you this way, so with at least one point in Necro and Hydro you can burn, bleed and/or poison several enemies and heal for dozens of hp/turn from status effects alone.
