Thanks for everyone's help. In the end, it was Brian finding the Truecharge 40 amperage usage that made me decide to install a separate circuit. I still wonder where all those amps go.....
Brian's marine switch idea, or everyone's suggestion of a bus bar sound like viable options for the future if I run into this situation again. Another option that I figured out (but didn't really like) was to install an outlet, and then wire plugs onto the ends of the charger and water heater. I could then just plug them in, or not, to turn them off or on. Didn't seem elegant at all and in the end the water heater DOES have a 1500W element and thus doesn't play well with others.
Part of why I didn't want to run more wires was apparent today as I was drilling a hole for the new wire runs right next to the current wires. My cordless drill broke long ago and I'm poor and haven't replaced it, so I was using a high speed plug-in drill. Well, it wasn't as easy to control and I did exactly what I was afraid of. I cut into the existing wiring.
Not only that, but I did it in the SHOWER of all places where moisture is likely to invade any repair. I spliced the wires back together with heat shrink butt connectors and then shelled out $40 (I hate west marine) for a length of 3/4" heat shrink to go over the whole lot. What a PIA....
Since I was running wires, I decided to run a few more and now I have more circuits and outlets than any boat could ever need. I can't believe I cut into the existing 110 wiring... Lucky for me I disconnected the boat's 110V at the dock and ran a separate extension cord directly from the dock, just in case. Perhaps I shouldn't have, maybe the thought of death by electrocution would have made me more careful.
To think, just 6 months ago I was doing brain surgery on children and now I can't even keep from shredding wires that are 1/4" away.
Gotta admit though, they had MUCH nicer drills at the hospital.