should i install electric water heater to save $$?
I had a similar issue recently, when the coil for my indirect hot water heater, fired off my fuel oil fired boiler, developed a leak. It would have cost approximately $800 to replace the coil in the boiler, with no guarantee of longevity, so I capped off the coil and installed a heat pump type electric hot water heater.
As for saving money, it is hard to envision saving any money in your situation. Typically the cost per BTU for electric hot water exceeds the cost per BTU for oil fired hot water, even accounting for high fuel oil costs. Plus you will need an up front expense to install the electric hot water heater. In my case, it was a relatively easy decision, since installing a new coil in the boiler looked like it was going to be a temporary fix.
Now that I have electric hot water heat, I turn the boiler off at the end of heating season. You can leave the water inlet valve to the boiler in the on position or you can shut it off, I don't think it much matters, since you will be using no boiler water during the summer, unless you have a leak. As to what you do in the winter, well that will depend on whether you keep the oil fired hot water system installed, in which case you are going to have two different hot water systems, and the plumbing and options get more complicated.