Well Nutz, it's gone from confusing to boggling. I went out to the garage tonight to try Mud's jumper idea thinking it would immediately yield some kind of results - either there is a ground issue or there isn't. Too bad everything just worked when I turned the key! I didn’t touch anything first. DAMNIT! I turned the key to 'accessory' first and the radio, blower and everything worked flawlessly. Where they wouldn't even turn on before, now everything works like normal. The only slightly weird thing was when I turned the starter, it clicked, paused, then turned at normal speed and the engine started right up. Then, each time after that, the starter spun right away and totally normally. (Note: Forgot that water was going to spray wildly out of the exhaust all over my garage walls including the baseboard heaters. It did – big mess.)
So, I checked the voltage across my battery terminals and it was steadily 12.24 to 12.26 volts. I put the charger on it and let the radio run a while just to see if it would act weird. It didn't, and the charger finally acted normally. Before, when I ran the charger, the guage always said the battery was full and it wouldn't plunge down when I turned the starter (even though the starter would only click). Today, when I'd turn the starter, run the blower or any other thing using a fair amount of power, the needle would dive like normal.
Now I'm more confused than ever! The boat hasn't moved from its spot in the garage and the weather hasn't changed significantly enough to make a difference. Temps are similar and humidity is slightly higher. WTF