I'm surprised you didn't have the Check Engine light on before you had the running problem. The water in the shut-off valve isn't uncommon on earlier SLK's. The vent tube on the charcoal canister shut-off vave was modified by MB (I don't remember exactly when), to minimize the possibility of water entering the shut-off valve/canister. If possible, look at a new shut-off valve, and see if the black plastic vent tube is longer, and pointed in a different direction, than the one on your car. If so, you've got the early version. If the charcoal canister is soaked, you'll need to replace it as well.