I have gathered from poking around on the internet and from some other sources, the following:
Shellac material, in its least processed condition, naturally contains a certain amount of wax. Much shellac, though not all, is de-waxed as it is processed for sale. It comes in different colors, ranging from dark to virtually clear, also called white." (White shellac is not to be confused with the shellac-based primer that Zinser sells which is actually white in color.)
It seems that de-waxed shellac is sensitive to moisture, whereas waxy shellac is water-resistant or water-proof.
As near as I can tell, clear or white shellac, and light-toned shellacs (blonde) ares always de-waxed, and therefore will be affected by water -- turning milky white if wet.
It seems that orange shellac is usually waxy, but may be dewaxed -- therefore, orange shellac that has not been de-waxed is the stuff to use on a canoe bottom. Clear or white shellac is not suitable. And I would guess that any other dark or darkish color, such as "amber," might be okay, as long as it has not been de-waxed.
While alcohol is the usual solvent for shellac, I have seen assertions that canned shellacs may have water added as well as other materials for preservatives, which may affect its ability to withstand water.
Alcohol comes in various flavors -- ethanol, methanol, isopropyl, denatured, rubbing (these terms are not mutually exclusive). Many, if not most, commercial alcohols contain some water. Because alcohol is hydrophilic (that is, it attracts water), it is difficult, though not impossible, to obtain 200 proof (that is, 100%) alcohol of any kind -- 190 proof seems to be the usual commercial upper limit. Rubbing alcohol found in drug stores varies between 70% and 90% alcohol. I'm not sure of the proof level of the alcohol solvent usually sold in paint and hardware stores. Denatured anhydrous alcohol, 200 proof except for .5% added poison to make it undrinkable, is available, but it is quite a bit more expensive than other alcohols.
So -- if I were using shellac on a canoe bottom, I would use orange shellac, and make sure that it has not been de-waxed. Cans of pre-mixed shellac usually state if they are de-waxed, but when the can says nothing, I'm not sure of the waxiness of the contents. It does not seem necessary that the alcohol solvent be completely free of water when used for this purpose, but it also seems that the less water, the better.
Sellers of flake shellac, which you dissolve in alcohol yourself, usually make the wax condition of their product quite clear.
As with much information gathered from the internet, there is no guarantee of accuracy, and if anyone has better or more correct information, I would welcome it.