I have a 86 Bahner 21' with a Berkeley jet and BBC 454.  Just got the engine back from a rebuild and would like to install an exhaust diverter system while I'm putting it back in.  I like to hear my radio when I'm idling around the marina.

I purchased some used,  solenoid actuated, exhaust diverter valves that are kind of a Y configuration.  Coming off my logs and snails is a 4" exhaust hose that heads out the back transom above the water line to the exhaust tips.   I have enough room to splice in the diverter and run a 3" hose below the boat but I'm concerned about that and don't want my boat on the bottom of the river or blow my motor by pumping it full of water.  Also wondering what tips to use if this isn't a stupid idea.

Is this a bad idea?  Anyone have any experience with this?