I assume you just have straight exhaust tips now.
As far as quieting it down, it depends on how quiet you want it and how much you want to spend. Installing Silent Choice will probably run you about $1500. You have to pull the engine to put in a Y pipe.
The least expensive alternative would be some Drew Marine Shotgun Mufflers. They won't make it quiet, but will take the edge off, especially at idle. Also, there is no HP loss with them. The inserts will cost you HP and do not work as well.