I agree, the high voltage battery is low down under the boot/back seat and is 300volt on hybrid and 360volt on PHEV and BEV. Not something you want to get wet.I wouldn't go through anything more than touching the bottom of the sill, to much high voltage cabling in the car
There are air intakes in the interior of the car to draw cool cabin air to cool the battery. It's located on one side of the back bench area right where your leg would be in the hybrid version, not sure about the other versions though.There's an interesting video on You Tube where a guy inspects the underneath of an Ioniq EV. The whole thing looked virtually flat, and completely shielded in panels. I think this is primarily to maintain excellent aerodynamics, but it must offer a lot of protection against weather and water ingress. However, I'm not sure if there are any small air intakes to cool the battery packs, and don't fancy driving through deep water to find out!! I will check it out when I have it up on the ramp next.