Yes, you will need a new circuit for a new water heater even a small one. I think water heaters are sized on the number of showers. That is the biggest factor.
120V means the type of voltage. When you say a new breaker your talking about handling load/amperage/available power to run your water heater. On your water heater is a label that says how much power the unit draws. In the national electrical code handbook is a section(s) that tells you what size breaker and wire is required depending on your current draw or required power, etc. Having the right voltage is only half the equation.
Master Electrician, Air Conditioning & Heating Repair Company Owner. www.TECertifiedElectricians.com