The AIR pump supplies compressed air directly into the exhaust manifold for emissions reasons. The added air re-burns oxides of combustion and any unburned fuel that enters the exhaust manifold. The cause of failure could be a bad dirverter valve in top of the AIR pump.
The AIR pump comes on and blows fresh air into the exhaust manifolds to help reduce emissions. It only comes on at times. It doesn't stay on all the time. You are probably blowing the fuse because the pump is locking up due to water intrusion. Water gets in to the air inlet for the AIR ( air injection reaction) system and makes it's way into the pump.
Hope this helps!