Whether or not the water heater will come loose by hitting it from behind depends on the caulk that was used. Some of the polysulfide caulks have a very tenacious hold and you could easily damage the siding by forcing the water heater out that way,. I would recommend carefully cutting the caulk from the outside with a SHARP knife or box cutter. Go slow and pry gently. Its fussy work but you'll get there.
John