The amount of electricity required to make someone's heart sieze isn't actually that much. Arthur was going to give Gregor a higher dose anyway, considering his supernatural status, but still not anywhere near his full output.

That said, Evvie is a bit of a different problem than Gregor, since she's got that nifty energy armor which Arthur assumes is non-conductive (as he does with any armor that isn't obviously metal). Still, saving you does leave her in a temporary one on one, and as you said, that's not a good idea.

Edit: regarding re-absorption, the electricity doesn't just disappear if he releases it. It's still electricity and if he gives up control it will take the path of least resistance to ground itself, which in the absence of any other conductive targets in the water would be right back to Arty. So unless there's something else for that energy to get expended on, most of it would probably return to him. Though again, the amount he used was negligible when taking his full power into account so getting it back doesn't really affect anything.