I'm really surprised at some of the responses.
If you ALREADY have a 9.9 kicker, I would ABSOLUTELY use it for trolling.
There are so many advantages to using a kicker motor. I wouldn't even think twice about using the main if you already own a kicker.
Will a 90 troll? Yep. Is it good for it? Not really.....but not going to kill it either.
Mount the kicker and you are all set. And have a back up if you ever run into trouble on the big water.
S.