My recollection is that BOATING magazine's "cruise speed" for planing
boats is the throttle setting that produces the best fuel economy while
the boat is on plane. Typically, it is somewhere around 70-75% of the
engine's top RPM on a boat with a gas engine.

Actually that would not be correct. The greatest fuel economy with a planing
hull boat (eliminating the "dead idle" option) is achieved just after the boat
reaches plane.

Cruise speed is more RPM related. A speed where you do not stress the engine as
much as running WOT, but still achieve a reasonable speed. I like to say it's
the maximum speed that can be sustained for long periods without unduly
stressing the machinery. It's also a speed where engine temp
remains under control. Some if it is personal preference and there is no
"official" cruise speed for most planing hull boats- but I agree with Harry
that many boaters will "cruise" at 70 -80 % of WOT.