Update:

Thought I would first try a colder spark plug so I bought and installed
a set of the same type of NGK plugs I was using, just one step colder.
I also checked the engine timing...found it to be very close to spec,
perhaps at most a few degrees advanced (spec = 27-29 BTDC @ 2000 rpm)
and adjusted it to the low end of the spec.

I also installed my StingRay hydrofoil since I was going to be taking 3
other adults in the boat and wanted to insure as ease of planing as
possible.

Results:

Went out on the Sacramento River on Saturday. No pinging at all, even
at WOT (5000 rpm, 31 mph). As good as performance as before installing
new plugs and timing adjstment. Only heard pinging once...when I made a
tight circle at high speed...would suspect the extra load due to the
turning caused that.

I will be taking the boat out again this weekend with just 1 other
adult so I will remove the StingRay, but I can't imagine it had
anything to do with the lack of pinging.

I will also be checking the spark plugs after some more use to be sure
they aren't oil fouled, which I understand results from too cold of a
plug.