These big, luxurious baths that bubble and jet water for therapy and relaxation are becoming more and more popular - it is easy to see why. Most people rely on their current hot water supply, but many people soon find that a whirlpool waterheater is an essential upgrade.
This is for two main reasons. First of all, these things are big. Some of the two person whirlpools are maybe 80 gallons which is as big as a big tank. The second reason is that the whirlpool action - blowing air or water around - means the water cools more quickly than normal. Add to this that people tend to want to stay in the whirlpool for longer than in a bath, and you can begin to see where an additional water heater might come in handy.
In fact for some set ups, small tankless hot water heaters may be considered an essential rather than an add-on. Such a situation is when the whirl pool is installed above an unheated room - a garage say. This is becoming more and more common as people extend and build en-suite bathrooms.
There are many options for this additional heating, but a common choice is one of those tankless electric water heaters that are so compact and easy to install.