It’s a sunny summer afternoon, and our group is embarking on a tour of the Elkhorn Slough, a biodiverse tidal estuary on Monterey Bay. The slough is home to pupping harbor seals, endangered birds and the world’s densest concentration of southern sea otters – including a rogue otter that has learned a deadly new trick.

You may have heard of people kayaking through this slough, but we’re boarding a very different watercraft, a 37-foot, ADA-compliant catamaran called the El Cat. We get comfortable on padded seats and whip out binoculars and birding guides.

“This is an inland waterway and it’s calm,” says our Monterey Bay Eco Tours captain. “Feel free to stand up, stretch your legs and walk around. But please try to keep two-thirds of your child or husband inside the boat at all times.”

The El Cat is unique in this world. Company owner Wendy Kitchell custom-built it with brother Joseph Kitchell using a proprietary epoxy-infusion system. It’s so strong and lightweight, a cargo ship could run it over, and the broken pieces would still float. Not that that would ever happen on these protected waters, where the biggest danger is mispronouncing “slough.” (Hint: It does not rhyme with “plow” or “rough.”)

The arf-arfing of sea lions booms from the shore, as we enter the main channel. The vessel is silent and glides like a song. It turns out the El Cat is powered by what are essentially two golf-cart batteries. Being all-electric was important to Wendy Kitchell.

“I moved to Key West when I was 18, and started working on a charter boat on the third largest coral reef in the world,” she says. “I watched it disintegrate, die out right in front of my eyes. It was really sad for me to see that – whether it was a change in temperature or cruise ships or us putting snorkelers in the water with sunscreen all over their bodies.

“I wanted to figure out a way to have the least amount of impact,” she continues. “I’ve seen a lot of fuel oil go in the water in my lifetime on boats, and it was exciting not only to have a quiet motor with minimal impact noise-wise, but also not have emissions going into the water or the air.”

Elkhorn wasn’t always in the best of health. There were problems with agricultural runoff from dairy farms and environmental imbalances that let certain species run amok and others die out. But after much work, it’s become a model of recovery.

The skies and marshland are aflutter with more than 300 species of birds, many migrating on the Pacific Flyway. There are harbor seals doing the “banana pose,” with heads and rear flippers elevated to show they’re safe and happy. You might see alien-looking creatures that get swept in from the extremely deep Monterey Canyon – egg-yolk and lion’s mane jellyfish as wide as trash-can lids or a 6-foot mola mola (ocean sunfish) swimming on its side to give you the wonky side-eye.

As we head upriver, the captain introduces Rachel Clifford, the naturalist giving our tour. “Rachel knows a ton about otters,” he says. “And mud. She’s a marine-sediments scientist. We hope you have a lot of questions about otters, otherwise she can talk to you about mud for a long time.”

“The mud is like a layer cake, one of those rainbow ones. Some of it even photosynthesizes, turns green and other colors,” says Clifford. She pauses. “Ask questions, please! If left to my own devices, I will just talk about mud.”

We pass fat sea lions perched improbably high on docks. They can actually “jump-climb” up tall dock poles, leaping from the water and catching themselves with their flippers. “They always haul out on our boat dock, which is super helpful,” Clifford says.

Our nostrils clench as we pass a colony of Brandt’s cormorants. The diving birds make nests on collapsed docks, building them with algae, eelgrass and a lot of stinky guano for insulation. During breeding season, they rock a cobalt-blue balloon thingie called a “gular pouch,” Clifford says, that helps them attract suitors. “They mate for life, though sometimes there’s some cormorant drama, and males and females wind up going after each other.”

Next up are brown pelicans that likely just flew two or three weeks from Baja California. Then it’s western gulls, which have a red spot on their bills that chicks peck at to stimulate feeding. “Sometimes you see juvenile gulls who should know better doing that, too. Lazy!” Clifford says.

It takes only 10 minutes to encounter our first sea otter. This is good news for Monterey Bay Eco Tours, which promises on its website they “guarantee you see otters or your money back!” It’s a female that’s monkeying around with a clam shell on its chest. We know it’s female, because her nose has scars from males biting it during mating.

“Nobody’s really told male otters about consent,” jokes Clifford.

This mom turns out to be the vanguard of the otter battalion that owns the slough. Many are descendants of otters introduced decades ago by the Monterey Bay Aquarium, which helped reestablish the species in this part of California. Elkhorn is now at capacity with more than 100 otters – add any extras, and they just kind of leak out into other areas.

There are parent otters teaching pups how to forage, sleepy otters wrapped in eelgrass so as not to drift away and sun-bathing otters with paws raised in a “touchdown” pose. (Otters don’t have fur on their tootsies and this keeps them warm.) Not that you should, but you couldn’t throw a clam shell without hitting an otter on the head, and then it’d probably bounce and hit another otter.

Eelgrass depends on sea slugs to vacuum off its algae. But at one point, there were so many crabs eating the slugs, algae grew explosively and killed the grass. The otters’ reintroduction changed everything.

“They ate the crabs,” says Clifford. “It was a really exciting, unexpected success story. We call that a trophic cascade, when one organism being reintroduced increases the health of the entire ecosystem.”

You’ll find predators like kites and peregrine falcons here, but one otter has put the bird world on notice. “We have a sea otter that’s decided to take birds and eat them,” says Kitchell. “That’s not a normal occurrence. We call it ‘The Birdurer.’”

With our 90-minute tour coming to an end, we head back to the launch point, passing harbor seals periscoping with heads up in the water. “Ooh, I hear some harbor-seal sneezes!” announces Clifford, to a soft snuffling from afar. “It’s one way they expel nasal water.”

We disembark with new animal factoids to use at the next bar trivia night and a desire to visit again soon. We just hope it’ll be the same charming, wildlife-romping place in the future.

“With the flooding that happened in the Pajaro district this winter, we’re probably going to see an effect in the slough because of all the toxins that were introduced,” says Kitchell. “You’re going to see the impact it has on wildlife in the years to come. It’s just a fragile ecosystem, so we love to take people out to appreciate its beauty and diversity.”

Details: Monterey Bay Eco Tours offers tours ($32-$43) daily at 10932 Clam Way, Moss Landing. Make reservations at montereybayecotours.com.