Before deciding if you should install your water heater yourself or call a professional, there are a few factors you'll need to take into account.
The first thing to consider is the type of water heater you have. The installation will be more complicated than a standard storage-tank water heater if it's an electric tankless water heater.
You'll also need to consider if you're comfortable working with electricity. If not, it's probably best to call a professional.
Even if you are comfortable with electricity, it's always advisable to have someone else double-check your work to avoid accidents. The other thing to consider is the warranty on your water heater. Most manufacturers void the warranty if a professional does not install the heater.
So, if something goes wrong later on, you could be on the hook for repairs or replacement. Weighing all these factors suggests calling a professional for things to go smoothly.