1.If you find it difficult to start your water heater or if it stops heating very often, it is caused by insufficient water pressure. We'd recommend you install a booster pump as a solution.
2.It's better to have it installed at the outlet of your water tank to pressurize the water (hot or cold) to your home.
3. If it must be installed in front of your water heater, it's better that you choose a pump with a smaller horsepower to avoid damaging the heater with excessive pressure.