I have had my laptop immersed in water as well as our car got immersed in water (along with the car stereo)
Most of the damage happens if you start with out drying out th water.
With laptop I used hair dryer and it got rescued. However the water droplets were there at some places so the motheboard died eventually in three months time (not immediately)
With the car stereo, unknowingly we started the car once the car was out of water but could not rescue the stereo as it got powered on automatically (we could have detached the stereo which was not so tough but did not think about it)
So, in your case, I would open the case and wipe all the water and then put the amp in baking oven at a minimum tempertaure (enough to keep the droplets of water to evaporate) and then in Sun.
There is a very good possibility that some damage is already done thought after all this the AVR working.
Insurance option is the best if you have one or speak to your dealer about anything else that needs to be done for future failures.