Is it possibble to hook up a 240volt electric water heater when I only have a 100 amp service? It will use 18.8 amps

Absolutely yes.

The critical piece of missing information is the amperage required for the heater. You should be able to find it on the metal plate on the water heater or within the Owners Manual.

Are you replacing a gas water heater with an electric one? It's important to know just what your current load is, before you add a large load like a water heater. I would suggest you have someone evaluate your total current load before adding any additional appliance.