There is really only one piece of equipment essential for snorkeling. Technically, you don’t even need a snorkel. Or a swimsuit, for that matter.

But you do need a mask.

My friend Heather and I had long talked about conducting a REEF survey at Makua on Kauai’s North Shore.

So, a couple weeks ago, Heather headed out from Poipu, stopped by Java Kai in Kapaa for a caffeinated pick-me-up, made a slight detour to pick me up, and we headed north, past Kilauea where the famous lighthouse beacon stands as an historic sentinel, past the supposed most expensive grocery store in the United States in Princeville, past perhaps the most beautiful scenic overlook on Kauai, and into the surf town of Hanalei, where visitors were standing in lines for shave ice, smoothies, Bubba’s hamburgers, and additional caffeinated beverages. We crossed one-lane bridges, slipped by ocean vistas so close I could practically drag my fingertips over the water as we drove by, and we parked on the camouflaged public beach access road for Makua.

Makua is more commonly known as Tunnels for its underwater topography of caverns and caves that are much favored by SCUBA divers. The snorkeling’s not too bad, either—but only in the summer when Hawaii’s notorious winter waves lay down.

We picked the perfect day. The sun shone brightly as it angled for the horizon, my favorite time of day when shadows lengthen and the water turns inky blue in a picture-perfect place known for its backdrop of velvety green, cut-out mountains.

But when I dumped my snorkel gear out of its bag, I found my booties and fins—but no mask. And I’m not the kind who can go for an hour underwater with my eyes open with no protection.

Heather introduced me to REEF last year. REEF stands for Reef Environmental Education Foundation. As their mission states, the non-profit organization “seeks to conserve marine ecosystems by educating, enlisting and enabling divers and other marine enthusiasts to become active ocean stewards and citizen scientists.”

Basically, regular people like you and me conduct fish surveys, using a standardized method developed by scientists. We record the different species of fish we see and in what basic quantity. Then, we upload the data to the REEF website. Since 1990, the effort has generated 178,000 surveys across the United States, Central America and the South Pacific, and its data has been cited in more than 60 scientific papers and reports.

What makes REEF even easier is that you don’t have to go with an organized group or at a set time. You just snorkel or SCUBA dive—in what’s called the Roving Diver Technique—with a pre-printed sheet that already has the region-specific fish listed on it.

You don’t need much to be a REEF surveyor. Just a mask, really. And, then, only if you don’t have Heather.

Kindly, Heather suggested we take turns using her mask while the other noted the data on the survey sheet, and we headed for the water—after a detour to check on a sleeping Hawaiian monk seal down the beach.

When Heather wore the mask and looked at fish, Heather’s blonde head would pop up, and she’d report, “Butterflyfish. Fourspot. Few.” Or, “Wrasse. Saddle. Many.”

The fish are listed on our data sheets by their taxonomic genus, first, and, then, by species. Next to each are the letters, S, F, M, and A, which stand for single, few (2 – 10), many (11 – 100) and abundant (101+).

Sometimes I’d hear noises emitting from the snorkel, and I knew Heather was seeing something interesting. One of those interesting things was a juvenile Yellowtail coris wrasse, a look-alike of the more famous clownfish known to movie-goers as Nemo.

When I wore the mask and Heather recorded data, I’d lift my head out of the water and go, “Long, skinny. Not Needlefish.” And Heather would say, “Trumpetfish.”

And, then, because I was being lazy—because I had Heather—I pulled my head up and said something like, “Dark body with beautiful iridescent blue dots,” and Heather just looked at me.

“Shaped like a triggerfish?” she asked?

“Uh,” I uttered.

“Or a toby?”

“Uh.”

When I stuck my head back in the water, the fish was gone, and I had failed the first rule of fishwatching: Look for the identifying marks of the genus. That is, shape of the body; placement of dorsal, pectoral and pelvic fins; shape of the tail fin and snout; those kinds of things.

I hadn’t even taken a picture of it. But I do have a good picture of it in my mind.

There were no scalpel-like spines at the base of the tail as found in surgeonfish. There was no horn-like fin of a unicornfish. No ovate body of a butterflyfish. No bands or stripes through the eye or the tail. No especially elongated snout or puffy lips. No long whiskers hanging from the chin of a goatfish. That narrows the genera down—but only a bit.

In my defense, many fish don’t look like those you see in the fish ID books. A female can look totally different than a male. And a juvenile different than adult. And don’t get me started on those females that change into males, typical in wrasse, with color variations throughout the process.

A couple days later, Heather texted me: Maybe an Ambon toby?

I’ve scoured my books. At first, I thought it was a female Black triggerfish. But, now, I wonder whether it wasn’t a small Peacock grouper—before its spots fade and a series of light vertical bars appear on the rear half of the body. Or, maybe it was a female Shortnose wrasse in the process of transforming into a male.

Truth is, I really don’t know.

Here’s what I do know: It’s now known as the Mysteryfish.

Heather and my partnership identified 12 different genera of fish for a total of 34 different species. It was a good day. The more we looked; the more we saw. That’s one of things I like about REEF surveys. But, now, all I can remember, the story I keep thinking about when I recall our outing—besides me forgetting my mask—is the Mysteryfish.

It’s always the fish that got away.