In calculating system natural frequencies and modes of vibration, CAESAR II does not consider the environment. The calculated system response assumes the piping is is NOT surrounded by water.
(Not to hijack this thread, but ...
Other (temperature-related) conditions may also come into play here - 1) the operating modulus of elasticity, try running Eh rather than Ea (Ea is the code's "reference modulus of elasticity" or E at 70F. Ea is specified for expansion stress range calculations. 2) the "tuning-a-guitar-string effect". The frequency response of a system is sensitive to any pre-tension in the system. Our analysis is based on a totally relaxed (otherwise unloaded) system.