One of the perks of being a Californian is that Hawaii is a quick, often affordable getaway: Without the need to escape frosty weather, we’re free to visit during summer, when the surf lays down flat and the price of hotel rooms plummets.

So for three decades, I’ve returned, year after year, to Big Island, swimming at the same bare-bones beach, which locals identify only by the number on a nearby highway mile marker. I measure my life by those trips, as surely as watermarks measure the tide. In my 20s, obsessing over career moves and noncommittal men, I escaped there with a girlfriend. Later I honeymooned there with my husband. These days we travel with our teenager, who swims confidently away from us, out to sea.

Along the way, I’ve amassed a library of books on identifying the psychedelic-hued fish, eels, octopuses, rays, turtles, nurse sharks and coral that live beneath the waves, keeping lists of what I’d seen on each trip. Gradually, and especially in the last five years or so, the variety and numbers on those lists have contracted.

At first I thought it was my imagination, but this summer there was no denying it: I felt, abruptly, like I was snorkeling through an underwater desert. Most of the coral had turned white, a sign that it was in danger of dying. Entire species of fish had vanished, and those that remained — like Hawaii’s tongue-twisting state fish, the humuhumunukunukuapua’a — were sparse, barely a classroom’s worth, let alone a school.