Good thread, interesting question and thought-provoking, thoughful answers.
I think there are a couple of further points worth floating here.
Firstly there's the important question of the direction(s) of energy propagation. This is not easy to determine without access to external information sources, because most of the energy transported by a complex seastate is contained in the longest wavelength ground swells arriving from far away, which are very difficult to detect and differentiate from what is built over them.
Unless they're abnormally high, (generally they're long and low) both their wavelength and their direction are effectively camouflaged by shorter wavelength swell trains from different directions. These in turn are overpainted with local wave trains of much shorter wavelength and with much steeper flanks, and whose direction reflects the local wind
If the direction of the first-mentioned long-period swell trains is alongshore rather than onshore (as often happens on the east coast
of NSW), the shallows will not be a bad place to be, in fact arguably they'll be protected to some extent from the energy of these trains, which cannot travel far in shallow water
. It makes sense that it would be a habitual practice in such regions to sail with 'one foot on the beach'.
On the other side of the Tasman, there is a long fetch for the prevailing westerlies of the roaring forties. The entrained energy is directed onshore onto the west coast
of the South Island of NZ, where there is no enthusiasm whatsoever for travelling close inshore.
There may be even more energy arriving from distant storms in the Southern Ocean, travelling alongshore (ie from the SW), but the energy from the west /norwest is sufficient, and from far enough away, that it is often problematic well offshore.
The second question: at what depth
? is complicated by factors others have mentioned, particularly the rate of shallowing, and the presence of ocean and tidal currents which can cause upwellings which have major effects on seastate, often eclipsing any rules to do with local depth.
But any rule
is also rendered difficult to apply by the difficulty mentioned above: discerning the period (or wavelength) of the longest, lowest component swell, which may contain the bulk of the energy.
Because it's THIS period which has to be plugged into the formula (assuming a gradual change in depth)
I think the difficulty of doing this might help explain why, among practical experienced sailors, the theory has become discredited, because the period they've inferred has not been nearly long enough to match the observed effects on the wave energy.
Add to this the fact, as has been observed by previous posters, that the rule
relates to the depth when waves start routinely breaking forwards (rather than spilling backwards) without any other provocation.
This is is an order of magnitude more consequential than the changes which interest us as sailors - most of us have never
seen a large wave breaking forwards offshore, and would not wish to (except on youTube, if we're given to masochism).