First, I always feel we need to understand how something works, if, we going to buy and use it.
This link show a simple on Hot Water Heaters;
Now, once we see the concept used (Very Basic) then we can move on to a faster version, the tankless.
I have worked on these before, both in home use and commercial use. BIG 1,000 gallon tank with this small Instant heater not much larger than a house unit. How this commercial unit worked with the use of circulating pumps and as the Large tank cooled the small unit heated the water as needed. I worked directly with the Manufacture for a City Unit as this was "new school" to myself.
I wanted to find you a picture, maybe later or someone else can. But, in the link I give you see the water tank? What they do is Coil a pipe for the water to replace the tank. As the water flow through the coil it becomes heat by Flame/electric (You can even use Solar or I can and will try to explain this later). As mention already, we know Fire can be dangerous (Proper ventilation stuff as fire needs oxygen to burn). Electric may not heat as efficient as Fire (Any flammable gas or wood will make fire). The electric is more safe as it not produce CO2 or burn up the oxygen as a fire will.
OK enough on basics, you can learn from there and may already know all this.
For the heater to work we have to provide time to heat. This why we not use just a straight pipe as the water not in the Fire/Electric long enough to heat, so, we coil this. A putt-putt boat is a good example (A toy many made as a child). So, not sure I going to answer your question, but, electric not seem to be a very effective heat source for hot water, unless you running a generator or on shore power. I like hot water when I off those life-lines.
Here how I did this.
I had 20 acres of land and it was far from a Fire Department. I tell everyone, Have enough water hose to reach where you need. I bought a 100' "Black" rubber hose. When I would be clearing my land, I make a brush pile to burn and have my hose close with water faucet on and a sprayer on the end to turn off and on. What I noticed was when the water was off, in full sun. it became hot inside the hose and when I turn it on it stay hot until the cold water from the faucet push the hot water out. Well, that how those House Roof and Tankless Hot Waters work! So, I had to do some math. Depending on my inside hose diameter, how long of a hose did I need for a specific amount of hot water before the cold come out?
Now, I can lay a 100' "Black" rubber hose (Green may work) on top of my boat and have enough hot water, but, this not too efficient with a hose running everywhere to trip over. Years before all this I had done a research paper in college on Solar Ovens (Way before people ever heard of these, I have mentioned for the women here, I not old, I am 35 and been 35 for 15 years
). When I coiled my hose and put it inside my Solar oven it became a small version of the Tankless Hot Water Heater!
We see this concept on the top of a house, it inside a sheet of glass or whatever!
Now, this not the best water heater, as mentioned a person need to know how hot and adjust the cold water.
My Camping Trailer help some. When on a contained water source (Maybe 30 gallons) I learned to conserve water. So, when taking a shower I used a hand sprayer and wet myself and turn off the water. Soap up and then turn on the water to rinse. I not sure of others here, but, I do the same on a boat unless I on a life-line to shore.
So, my little garden hose work for me and it not have such a pressure increase as to worry with a release valve as we see on some applications.
Well, I not sure I help you, but, sometimes want to interject other ways we can approach the situation. I will assume the Electric you mention will work, but, it up to you to decide the cost against other alternatives. Good Luck and maybe one day we run into each other in the open sea. "Ramming Speed"!