Although they are very similar to the things that we use for pools, electric spa heaters are a little different and need some additional precautions.
The main thing about spas is that the temperature is high. Pretty obvious. The maximum temperature should not exceed 104 degrees Fahrenheit - this is Federal law. It is also very hot. Most manufacturers and medical experts recommend that the maximum you spend at this temperature is 15 minutes, then take a shower. This advice is for people in good health - the young, elderly or infirm should not use the spa at this temperature.
So, all that said, an important element of the spa heater is a really good quality thermostat. And though they are heating a smaller amount of water, these heaters need to do it quickly and to a high temperature. What is needed is a purpose built unit.
People tend to be a little too laid back about spas - maybe because they are quite small compared with pools. But there are at least as many precautions to be observed - there is the real risk of scalding injuries (seldom an issue in a home pool), and they are large enough to drown in. Spas should not be taken lightly.