Yes, travel insurance is genuinely useful for a trip to the Blue Lagoon in Fiji's Yasawa Islands, and the honest reason has more to do with getting there and back than with the lagoon itself. This stretch of the Yasawas is reached only by boat, typically the Yasawa Flyer catamaran or a chartered vessel from Port Denarau near Nadi, or by seaplane, and rough seas or weather can delay or cancel a crossing with little notice. The lagoon's calm, famous waters are themselves low-risk; the exposure is almost entirely logistical.
Medical care in the Yasawas is minimal, limited to basic first aid at resorts at best, and anything serious means transport back to Nadi, the region's real gateway and home to the nearest sizeable hospital. For genuinely serious cases, onward evacuation typically continues to Auckland or, depending on the case, Australia, since specialist treatment beyond what Nadi can provide means leaving Fiji entirely. Confirm your policy covers inter-island transport as well as international medical evacuation, since the transfer off the Yasawas is the step most likely to involve real time and cost.
Cyclone season, roughly November through April, is a genuine consideration for any Yasawas itinerary, and storms can suspend the ferry and seaplane services that are the only way in or out, stranding visitors on the islands or on the mainland for days in a worst case. Trip-delay and cancellation cover is worth checking closely if your visit falls in this window, since resort bookings and boat schedules do not always allow easy rebooking around a cancelled sailing.
Keep any resort or clinic paperwork, boat-operator cancellation notices, and receipts for unplanned accommodation if a crossing is disrupted, since documentation of the disruption itself matters as much as any medical receipt here. Many policies treat weather-related ferry or seaplane cancellation as a standard delay claim, so check the wording for how it defines a disrupted crossing before assuming automatic reimbursement for a stranded night in the Yasawas.