On Aug 16, 9:45 am, Robert Allison wrote:
[...]
Then the rains came.[...] The pedestal was under about 15' of
water, yet did not trip the breaker at the main service. The owner
called me and asked if this was dangerous and I said yes, turn off
the breaker at the breaker box, which he did.

My question is: why did the breaker not trip when the
disconnect was submerged? It is not waterproof by any means.

Because water is not conductive enough to draw 15amps. It was however
very much a hazard to anyone nearby, since a tiny fraction of an amp
can kill.