I ran the boat (336) on Saturday with everything working just fine, and then Sunday morning when I started it I noticed that both the left water temp gauge and the voltmeter were both inop. I didn't have time to mess around with it, but on the way home I studied the wiring diagrams to see if there was something in common between the 2 gauges and I don't see anything shared.

Next time down I'll pull the dash and start looking, but does anyone have any ideas?

Yup, I do!!
I can almost guarantee it's a bad ground at the gage or at the terminal board behind the dash. Been there Dude. I'm confident you'll find it and it's easy to fix. Clean 'er up and you'll be back in business.

Like Third said , ground is common.

I think I was in Thirds dash as well?!?!