Try letting it run longer. There is usually not a lot of water pressure at
idle and water may not have fully filled the motor. The fact you have water
at the pump will prevent the pump from running dry therby preventing the
normal damage to the pump caused when you run with no water. Other than a
dry running water pump, the engine will not be damaged from a lack of
cooling water, should that be the case, for some minutes and if you are in
doubt, remove the engine cowling and feel the upper cylinders and shut the
motor off when you feel the block hot to touch. So bottom line run it a
while, don't go over and idle or fast idle at most.
Good luck,

--
Ron White
My boatbuilding website is:
www.concentric.net/~knotreel