Most people who have lived in Hawai‘i for a while know that Diamond Head crater got its English name from the sparkle of worthless crystals in its rocky exterior, and not from real diamonds. And many know that it got its Hawaiian name, Lē‘ahi, from the resemblance of the crater's ridgeline to the dorsal fin of a tuna. But the name doesn't exactly mean 'dorsal-fin of tuna'.
I gave the etymology a closer look after finding a bogus etymology on the historical plaque that marks Diamond Head Lighthouse. Lē‘ahi does not come from lei 'wreath' plus ahi 'fire', and thus has nothing to do with Hawaiian practices of navigational bonfires imagined by the builders of the modern lighthouse. Haw. ahi 'fire' comes from Proto-Polynesian (PPN) *afi 'fire', a good and widely reflected Austronesian root. In Numbami, a language I studied in Papua New Guinea, the word for 'fire' is yawi (with the /w/ slightly fricative before front vowels). Even highly aberrant Yapese seems to have a cognate, nifiy 'fire'. But Haw. ‘ahi 'tuna' comes from Proto-Nuclear Polynesian (PNP) *kasi (with *k > /‘/, *s > /h/), which seems not to be so widely attested beyond Polynesia.
The first part of the compound is trickier. According to Place Names of Hawaii, lē comes not from lei 'wreath, garland', but rather from lae, which in place names usually translates 'cape' or 'point'. However, the Hawaiian Dictionary lists a wider range of meanings: 'forehead, brow; cape, headland, point, promontory; wisdom' (< *la‘e). So some of the tourist literature now translates Lē‘ahi inappropriately as 'tuna brow' or 'brow of the tuna'. Tuna Head(land) would be just as accurate, but Cape Tuna or Tuna Point would be more in keeping with the glosses in Place Names of Hawaii. (I think we can also rule out Tuna Wisdom!)
UPDATE: But what about Aku Head?
There used to be a well-known radio personality in Hawai‘i who called himself "J. Akuhead Pupule" (= 'J. Tunahead Crazy'), reputedly after being called as much by irate listeners. I'm not sure why aku-head would be a sharper insult than ‘ahi-head, but it might be because aku is the Hawaiian name for the bonito or skipjack tuna genus Katsuwonus, whose name derives from Japanese katsuo, which in Japan is usually dried into a woodlike block, katsuobushi, from which flakes are shaved off for use as a flavoring. So maybe akuhead = blockhead.
From my fieldwork experience long ago in Papua New Guinea, where I elicited far more fishnames than I had ever heard of before, I would guess that Haw. ‘ahi matches pretty well with the genus Thunnus while Haw. aku names tuna of the genus Katsuwonus (and perhaps a few other similar genera).
In checking Wikipedia entries for other members of the family Scombridae (tunas, bonitos, and mackerals), I see that the one for wahoo, Haw. ono (< PPN *‘ono), offers entirely unsupported speculative etymologies for both names. Wikipedia should not confirm one person's speculation with that of another. Leave that to the news media.
UPDATE 2: The cape at the southwestern tip of O‘ahu—a long, flat counterpart to Lē‘ahi at the southeastern tip—was for a time called Barber's Point, but has now reverted to its Hawaiian name, Kalaeloa 'The Long Point'.
A reader asks why I don't simply consult a fluent native speaker of Hawaiian. One reason is that native speakers with encyclopedic knowledge are extremely rare in any language, but especially for Hawaiian these days, where most fluent speakers learned the language in a classroom, not from their parents and grandparents long resident on a plot of land. (In fact most have lost the lands of their ancestors.) Another reason is that local residents are usually content to offer folk etymologies and wild guesses when asked about anything not utterly transparent linguistically and historically. At least that is my experience from incessantly asking just such questions of local drivers, store clerks, wait help, baristas, sushi chefs, and random passers-by in my travels to odd parts of the globe over many years.