I use a heater like this in my Instructable on making perfect steaks.
I hook it to my themostat control project to maintain the temp.
Out of the box these heaters have a limit of about 90 degrees F. That's good for live fish, but I wanted to be able to hold water in a cooler at up to 150 degrees F. A computer controller immersion heater is $200+. A thermal probe and an aquarium heater are $60 or less.
This Instructable will show you how I did it.
1. Always use your head. House current, water and glass are involved. Lots of ways to get hurt.
2. Always keep stuff unplugged when you are working on it.
3. Read the warnings on the heater before you void the warranty.
4. Never dunk a hot heater into water. The glass will shatter.
5. Always use a GFCI outlet.