I'd disagree. If the boat / engine / trolling motor is not designed for saltwater, I wouldn't put it in there. Even with a good washdown and zinc anodes, saltwater will corrode things like switches, electrical contacts, etc.. over time, causing your boat, engine, electronics, etc.. not to last as long as they were originally intended to. Will you see an immediate result of this? Probably not, especially if you are giving it a good washdown. However, with the carpet on the boat, you will NOT get all of the saltwater spray out of the boat, no matter what you do (barring removing the carpet), and over a period of time, it will more than likely eventually cause some problems.
It's your boat - do what you would like to do with it. But from personal experience, living in Louisiana and Florida and fishing salt and fresh water (and I clean my boats meticulously), I would not recommend taking any "freshwater" boat in saltwater.
All the best,
Glenn