I think that you need to think about where and when you are going to use hotwater. If you are tied up in a marina with power, as is the case for most liveaboards then a tank heater makes most sense and is easiet and cheapest to install. If you are on the hook or rarely use your engine then the tank ones are not very useful. So if your cruising is ICW you are fine; if you are sailing long distances offshore you are out of luck unless you have a genset or need to run the engine to charge batteries (which you probably don't with solar/wind).
I have had a couple of boats with tankless, gas-powered heaters and quite like them. Caveats though, you need a marine unit not a domestic one (they exist, and are quite common in Europe, but not cheap, you need a suitable spot to install one - Hinterhoeller boast typically used them and they built the galleys so the unit was at eye level and handy ( you lose some good cabinet space here but it was easy to then build in the flue. I guess what I am saying is that they work better when they are designed into the boat not added on. BTW, I don't see a particular advantage to the electric, flash heaters - if you have 110v go with a tank. Final comment about gas heaters, ours had a pilot light (Paloma, no longer recommended for marine) but we never left the unit going between uses since it was very easy to start the pilot each time.
Back to Grenada in early December. Not sure I will remember how to sail. Will spend the winter and early spring in the Caribbean and then head to Bermuda and the northeast US. Still trying to decide if we will bring the boat to Canada, either in 2015 or 2016.