I have had my vx swimming with water 6 inches up the windshield. I have a snorkel so it can still breath never had any issues with idle and random acceleration. I however have sealed my fuse box intake coil packs pretty much anything Electrical I could think of.
However any leak in the exhaust while under water can allow water to back up from the tail pipe because the pressure escapes through the leak. It only takes a small amount of water to shatter a nice hot converter the are porcelain after all and yes they do have platinum in them. Water up the tail pipe is a ricer trick to de cat a car. Couple of things about a clogged/shattered cat if its shattered you should be able to shake it and hear it rattle unless it all fell out. A clogged cat can leave the exhaust smelling like rotten eggs.
A red hot cat can also indicates you could be running very rich. Since the extra heat in a cat is caused by the chemical reaction burning the fuel. Have you checked to see if you have water in your spark plug wells
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