Yes, travel insurance is close to essential for a trip to Ha'apai, a remote group of islands in Tonga with very limited local medical infrastructure. The honest core of the answer is evacuation logistics rather than any dramatic on-the-ground danger - if something serious happens here, getting full hospital treatment means leaving the islands entirely, and that takes both time and money without the right policy. Ha'apai is genuinely remote even by Tongan standards, with far less infrastructure than the main island of Tongatapu.
Serious cases in Ha'apai are typically stabilized locally if at all possible and then evacuated first to Nuku'alofa, Tonga's capital, with further specialist treatment usually meaning a transfer on to Auckland, the real regional hub for the South Pacific. Confirm your policy explicitly covers medical evacuation between islands and international repatriation, since a policy that only reimburses local clinic visits misses the part that actually matters here. Budget for the possibility that a serious incident becomes a multi-leg journey rather than a single transfer.
Diving and yacht sailing are genuine draws in Ha'apai, popular with cruising yachts on Pacific passages and with divers drawn to its reefs, and both carry real risk in a place this far from emergency services. Cyclone season, roughly November through April, adds a separate layer of risk - storms can disrupt inter-island transport and force evacuation plans of their own, so confirm your policy covers diving and cyclone-related trip disruption specifically.
Keep any clinic paperwork, dive-operator or yacht-charter incident reports, and receipts for emergency transport if you need to file a claim, since documentation is scarce and easy to lose track of in a place this remote. Many policies require notification before major expenses like evacuation are incurred, so check the claims process in your policy wording well before you travel rather than after something happens. It is worth photographing any documentation on the spot, since paper records can be difficult to replace later from outside the islands.