Immersion heaters require no plumbing, simply plug into 120v or 240v GFI outlet and suspend the element in the water. 120v Immersion heaters are best for use with smaller or portable baptistries while 240v heaters can be used on larger pools.
Circulation Heaters give more efficient heating of larger baptismal pools as the pump circulates the water throughout the pool. Circulation Heaters should be used if you do more than 4 baptisms a year and installing plumbing and cutting holes in the tank is not a problem.