That 115 lbs is dry weight. It is an unwieldy thing to put on or remove from the back of my boat without help. I have an 18 HP Nissan and its basically the same motor if you're considering a Mercury.
The more I think about it however, and the type of fishing I do, a 9.9 HP would do me just fine. Even on a bigger lake. I have a 14 foot fiberglass boat.