Is Thunder Damage % Special?

I know water damage affects how good heals are. But I’ve seen items with negative thunder ID’s, like Nepta for instance, when it should have no reason to (except maybe to reduce teleport damage?).
Does thunder damage % affect anything, besides thunder damage, obviously?

(Also, as a side note how much does water damage affect heals? Is it (base heal)*(water damage %)? Or does it have diminishing returns like loot bonus?

EDIT: just did what I hope to be a good test by manually changing water damage to 100%, but it only increased my heals by 50% so I guess it does have diminishing returns)

 

1. no

2. 0.5% heal effectiveness for each 1% of water damage, capping out at 75% with 150% water damage