Sluggish, mis/back-firing after going through puddle

I have a '96 British Ford Galaxy (which is a minivan) that has just over 100k on the 2.3 ltr engine; but that may not matter. Shortly after I drove (quicker than normal) through an icy cold puddle this week the performance began to drastically diminish. It acted very similar to when the catalytic converter went; sluggish, misfiring, backfiring and generally sounding like a Harley Davidson with straight pipes. Only unlike when the catalytic converter gave up the ghost, the noise wasn't from the box under the drivers seat, it was coming mainly from the tailpipe. I took it to an exhaust specialist place and they put it on a rack and said it was something wrong with the engine. I've had the whole exhaust system replaced within the past month so I believed him. My question is, if the engine is, for lack of a better term "waterlogged" is it an expensive fix and is it something a layman like me can do in a couple of hours?