As far as saltwater, I don't know. I'm sure that even the "saltwater" series are still going to need extra maintenance based on my experience with saltwater.
More importantly, go for the weakest you can get, it will last much longer on a battery (which will weigh more than both of your kids combined, and will not be cheap, and the charger wont be cheap, and the battery box won't be cheap, not when you add it all up).
The problem with the trolling motor industry is that the manufacturers kept trying to outdo each other in thrust pounds, so while you need a 15 or 17 lb thrust that will run all day long on a battery and give plenty of thrust, you will be hard pressed to find anything under 8 jigamillion pounds of thrust. It's just the industry.