Thanks for all the input folks. My issue is saving water when waiting for the hot to arrive at the sink, not the OP's issue; so I sort of high jacked this thread, and now I regret that, but........
Anyhow, to wrap up my issue part of thread, here's what I've concluded (in a nutshell.....what I want won't work). The options seem to be:
1. Use a small "insta-hot" unit under the sink (like many residential houses use except smaller) that stores 1 to 2 gallons of hot water directly under the sink.
2. A small tankless system that has no tank of hot water, but rather heats the water as the water flows through the unit on its way to the faucet.
3. Somehow arrange to have the cold water that now goes down the sink recycled back to the main water tank while you are waiting for the hot water to show up.
#1 -- is not appealing to me for several reasons. First, since the unit has a tank, these units are much larger than I wish them to be. For example, the Eemax unit thataway mentioned is 12.5" x 11" x 10"; the InSinkErater unit that Will-C mentions is 6" x 3.7" x 5.6". Basically these units are a sort of cube of various sizes depending on capacity. If I were to go this way, I'd go with the smallest tank I could. I did not find one that was as small as the unit Will-C found at a mere 2/3 gallon (thanks for that). A 4" deep unit might actually be acceptable especially since it only uses 750 watts (I could run it off the inverter to keep the water hot). Aurelia came up with the only 12v unit I've seen. I like the idea of 12v, but this unit's smallest dimension is 7.9"; plus it's expensive. Finally, IMHO the problem with these "an extra small hot water heat under the sink" solutions is the power drain it would take to keep the water hot during the course of the evening (or whenever). I could find no information as to how good the insulation is around these small tank units; but given their small size, I expect that one would be using a lot of power not just heating up the gallon or so of water in the first place, but also keeping it hot.
#2 -- this bring me to the tankless systems. No power drain to keep the water hot, one just uses power to heat the water one actually uses at the time of use......swell! The one I like is a Bosch Tronic 3000 US6. This unit (like all small tankless units) is quite thin: in this case 6.5" x 12.25" x 3". I've got room for 3 inches!! Unfortunately, I've concluded that this solution won't work either. The problem is that it takes a lot of power in a short time to heat water even a few degrees. Even the small capacity units (0.5 GPM) take 6000 watts. If I did the "drive a 240v unit at 120v" trick to get that down to the 1500 watts my 2000i could handle, I've also cut the possible temperature rise by the same factor of 4. My rough calculations indicate that using 1500 watts at 0.5 GPM would only rise the water temperature some 20 degrees....not nearly enough.
So I don't think my vision of "no wastage" hot water will work....except for thataway's simple and elegant solution of dumping the wasted cold water right back into the main water tank (duh, now why did I think of that!). I am going to look around the boat to see if there is some way to tap into the pressured hot water line near the sink such that the flow could be directed back into the main tank. I figure that if the infrastructure allows it, I could mount a valve in the galley; then through experience determine how long I should leave that valve open such that water of the right temperature is starting to arrive at the sink; then close the valve and open the faucet......viola...."instant" hot with little or no waste.
Later edit......This message got cross-posted with thatway's directly above.
