I flew airplanes with HF radios for years in Alaska, mostly Kodiak. They work fairly well at great distances, due to skip, but not so well at closer ranges. They tend to be relatively heavy, though units have gotten smaller in ensuring years, but you’ll also need a “serious” antenna and tuner system, which adds weight and complexity.
Granted, one radio can cover a lot of frequency spread, the challenge becomes providing a well matched antenna for that bandwidth. I used tuners, dipole antennas, and trailing wire antennas at various times. The trailing wire works good, but is a huge PITA, especially if manually extended/retracted. Dipole works great, but is more frequency specific, and limited. Worst performance was a big whip with a tuner.
What worked best for me was to call KMI, the Marine Operator in San Francisco, and have them patch me into phone system. Don’t know if that system even exists now.
Otherwise HF comm was very spotty and often frustrating at best.
Last several years I flew in Alaska, I used Satellite phones...Iridium system, with panel mounted units. We also incorporated a tracking system into these phones.
Trust me....I would NEVER even think about installing an HF radio in another airplane, IF I wanted to use it to communicate. The Iridium satellite phone system has just about replaced HF in the marine (and aviation) worlds. You might still talk to Ham operators....
Finally, that plane is going to be HEAVY on amphibious floats. You’re going to be wishing for more useful load, and trust me, an HF radio is NOT useful. Sat phones are light and compact. I wouldn’t even think of going any other direction.
MTV