Man...

Hehe, well, I think it is easy to understand that when you don't have a shield beneath it and when your car is hitting a big pot of surface water, it will be splashed or “poured” onto your right hot engine from below and everything under your hood will be soaked in this thick steam.

The key parts affected by water or vapor from the splashing IMHO should be the electric components, ignition coil is just one of them. Other possible parts are cables and spark plugs depending on their condition, sparks between distribution cap and rotor tip, the electronic chip inside of the cap (for some cars) depending on how tight the cap is sealed, and even the main chip for the car if the moisture is really too much. Another concern will be the car belt(s). When the belt(s) is wit, the performance definitely will be affected and in some extreme cases, it could break.

Of course the very first metal piece of exhausting system is kind of easy to crack when it is hot and water gets to it. However since it is usually sitting at front side of engine and it should be quite up there, no water should get to it while car is moving fast (it can have splashing) even without a beneath shield. Please also note for some models this piece has its own metal shield there. I don't think other part of exhausting system is that easy to get damaged by water or steam except rusting.

From the owner's original deion, I would say some parts of the electric system are affected by the vapor/steam. Considering the ignition coil is just one of them, I am not sure changing it will help. If I were the owner, I would check/fix the shield first no matter what, and it is easy and don't cost much. This is just my opinion.