I don't think a trailer boat will ever be in the water long enough in it's life time to create a problem. My sailboat has been in salt water for 30 years, and I have moderate barnacle and calcium build up inside the water hoses of my engine. I'll be trying "Barnacle Buster" soon, it looks like it would immediately take care of any Quagga mussel problem.
I just replaced my impeller on my Yanmar, I noticed the calcium build up (zoom in on the impeller.) When I squeeze the rubber hoses, the inside is lined with crunchy barnacle/calcium build up, and you can see the results in the debris in the impeller ( I did not remove the hose, as I was too afraid to remove the hose as I'd likely damage it and potentially sink...) I've noticed no overheating issues, but these motor's run cool with a 130 degree thermostats stock. Google Barnacle Buster, looks like a great product, I will let you know: