Dead Battery, Flooding, Honda Pilot 2012

I fled Hurricane Harvey for two days and left my 2012
Honda Pilot in my garage. After I returned, I discovered
that my battery was so dead that the dashboard light
would not illuminate when I turned the key. At first I
thought it was the water that killed it, but the waterline
only came up about 1/4 of the way up the tire. When I
jumped it, the car horn started blaring immediately. The
battery is still dead, and I can't shift it into gear to get it
out of the garage. What should I do? Why was the horn
blaring? No sensors were submerged, and the engine is
completely dry... Should I replace the battery or is this a
different issue entirely?