Catalina's Airport in the Sky is a small general-aviation airfield atop the island, not a commercial terminal, so the honest answer to whether you need travel insurance here is really about Catalina Island itself rather than the airstrip. Yes, coverage is worth having for a trip to Catalina, and the real reason is the island's genuine remoteness from mainland medical care, however short the crossing looks on a map.
Avalon, the island's main town, has a small local clinic for routine needs, but anything serious is typically stabilized there at most and then evacuated by boat or helicopter to Long Beach or greater Los Angeles on the mainland. Two Harbors, the smaller settlement further up the island, has even more limited services, so a serious incident there generally still routes through Avalon first. Confirm your policy covers medical evacuation and inter-facility transport specifically, since that crossing - not treatment on the island itself - is the part most likely to involve real cost and coordination.
Most visitors reach Catalina by ferry from San Pedro or Long Beach to Avalon or Two Harbors rather than by flying into the airfield, which serves general aviation and sightseeing traffic rather than scheduled airline service. Diving off Avalon's protected coves and hiking across the island's hilly interior are genuine draws, and both carry a modest layer of remoteness risk given how limited phone signal and road access can be away from the main towns; confirm your policy covers activity-related injury alongside standard trip cover.
Keep any Avalon clinic paperwork, ferry or helicopter-transport receipts, and documentation of any incident if you need to file a claim. Check whether your existing health cover extends to a mainland transfer originating from Catalina before assuming it does, and confirm your policy's claims process for evacuation costs before you travel rather than after something happens.