For years I have always up sized the breaker by at least 125% of the load on an Insta-Hot (sometimes known as a Tankless Water Heater). Now I find out that since it is considered a non-continuous load, I can size the breaker according to the load, at least according to one supplier and a company tech rep I talked to. This would have made things much easier if I had known this, but am glad I know now. Doesn't it seem to be pushing it too close to have the breaker right at the amp load? What about nuisance tripping in case of a weaker or eventually worn breaker? Factor in things like voltage drop or perhaps the device slightly raising it's ampacity over time due to wear or tear, it seems like a potential for at least some minor problems. Another question I have is whether a disconnect is required under the sink with the Insta-Hot. A well respected (and feared by some) inspector told me that if there was not a requirement on the blueprint for a disconnect, it doesn't need one according to code. I definitely always label the breaker in the panel and write on the insta hot what circuit and panel it is.
Around here they will usually want the local disconnect. Our AHJs do not think much of residential LOTO capabilities. Up to 20a this is usually cord and plug, 25a and up is going to be a pullout or switching device.