Well, the damage display is a bit buggy.
You get one listing of damage when you hoover over the plus sign, the other once you have acquired the skill.
I've been testing this and the damage you do is certainly a lot closer to the damage that's shown when you hoover over the plus sign (it properly IS that damage, but I don't have the patience to try it enough times to get a statistically relevant figure...).
My best guess is that the damage in the main display is minimum damage (damage you always do, even when the enemy is highly resistant to water), I've not tested this however (not a lot of enemies with a VERY high water resistance).
If you want a list of the damage of the lightning storm page (both main and plus display), check out
this page.
I don't know about the experience BUT you can try this out yourself easily enough. Just look at the experience in your inventory screen, kill a few enemies with LS and see if it goes up.