The boyfriend of a Northern California woman who drowned after she was swept out to sea Sunday by a " sneaker wave" said they took this same walk on the beach every day.

Susan Archer, 32, the fifth person to drown in Northern California this winter, was walking with her boyfriend, Sion Tallerico, and their dog, Trigger, along a beach in Shelter Cove when the rough waves took them by surprise.

"You have to be careful of big waves. We knew it and it just happened," Tallerico told ABC News.

"Trigger had gone around the corner … and when she went around the corner, I couldn't see her either. And by the time I was getting around the corner I saw her going out already."

Tallerico was injured when he was thrown against the shoreline rocks, but he was not swept up by the wave. Trigger was pulled into the water but was able to swim back to shore.

Archer's body was brought to shore after about a 45-minute search by boat and helicopter.

A "sneaker wave" is the largest wave in a series of waves brought on by dangerous weather conditions.

"If you're not prepared and the larger wave does come in, it can sweep you off your feet. The currents are very strong, and the water is really cold this time of year, which can take your breath away and pull you out to sea," Pamela Boehland, a Coast Guard spokeswoman, told ABC News.

In a statement, the Coast Guard warned that, especially during the winter months, Northern California beach-goers should never turn their backs to the ocean. A simple walk on the beach can be deadly for those not keeping a close eye on the churning seas, which whip up these sneaker waves.

Bob Gibson survived one in Oregon, despite being slammed by a huge log.

"My thought was, this cannot be happening to me," Gibson told ABC News.

Earlier this month, a man and his wife went into the ocean in Marin County to rescue their dog that had been overtaken by a wave - the man was swept away.

In November, at the beginning of this particularly dangerous winter season, a couple drowned and their teenage son disappeared while trying to rescue their dog, which had been pulled into the ocean.

"The pattern we're seeing is that these people are out with the dogs, so we really want dog walkers to be aware and protect their pets," Boehland said. "And if your dog does get swept away, don't go after them. It's dangerous, and they're usually stronger swimmers anyway."

Tallerico says people need to keep a watchful eye on the sea because a winter walk on the beach can be anything but peaceful.

ABC News' Erin McLaughlin contributed to this report.