JOHN YANG: Last year was one of the deadliest on record for migrants crossing the southern U.S. border.

Experts say tougher enforcement is pushing migrants to take more dangerous risks, like using the Pacific Ocean to try to get from Tijuana, Mexico to San Diego.

as Gustavo Solis of KPBS in San Diego reports.

The result has been a spike in drownings.

GUSTAVO SOLIS (voice-over): The sound of crashing waves and children playing along the Tijuana coastline looks like a normal day at the beach.

But at the U.S.-Mexico border there's more happening underneath the surface.

Here the wall sticks out more than 200 feet into the Pacific Ocean.

LUIS HERNANDEZ, Tijuana Lifeguard (through translator): The current is very powerful.

It pulls you out into the ocean in the United States.

They call it a rip current.

GUSTAVO SOLI (voice-over): Tijuana lifeguard Captain Luis Hernandez says that the metal posts from the border wall create a permanent rip current that pulls swimmers out into the ocean.

These waters are unforgiving, says Hernandez.

It's not like swimming in a pool where you can just grab onto the ledge whenever you need to catch your breath.

LUIS HERNANDEZ (through translator): There is no ledge here.

There's nothing to grab onto when you're tired and need to catch your breath.

GUSTAVO SOLIS (through translator): This stretch of ocean is particularly dangerous to migrants trying to swim around the border.

Over the last few years, Tijuana lifeguards have seen a record number of deaths.

They track rescues and drownings on a whiteboard at the main lifeguard tower, just five rescues in 2020 and only seven in 2021.

Then 59 in 2022 and 41 last year.

Hernandez says migrants crossed in one of two ways.

The first is just to swim around the border.

But fighting that powerful current is exhausting.

LUIS HERNANDEZ (through translator): They use the wall as a staircase and wrap their arms around the rails.

GUSTAVO SOLIS (through translator): The second is to wrap your arms around the walls metal beams and use shellfish growing along the bottom of the wall as little ledges but those shells are very sharp lifeguards often rescue migrants with cuts throughout their body.

LUIS HERNANDEZ (through translator): They are like little knives we rescue people with cut wounds all over their chest, arms and legs.

GUSTAVO SOLIS (through translator): Most rescues happen on the San Diego side of the border when exhausted migrants swim against the powerful rip current while trying to get back to the shore.

Whenever that happens, U.S. lifeguards respond.

Jason Linquist is the head lifeguard in Imperial Beach, just south of San Diego.

JASON LINDQUIST, Imperial Beach Lifeguard: If we get a call from the border patrol, they're watching someone hang on the fence or swim around the fence or in distress, we try to respond no matter what because it's in our city limit because if you look at the ID map, it goes all the way to the border.

GUSTAVO SOLIS: Lidquist says most rescues involve migrants who are not strong swimmers.

They often jump into the ocean with all of their clothes on and carry their belongings on heavy backpacks.

JASON LINDQUST: If the current pushes you against that fence, that's a dangerous situation because you can't get off of it.

So the fence is dangerous.

The currents are dangerous.

The circuit be dangerous.

GUSTAVO SOLIS (through translator): Imperial Beach lifeguards also notice more migrant drownings around 2019 after the border wall was replaced with the taller one.

JASON LINDQUST: We've had, I think in the last two years, way more fatalities from here to the border than we've ever had.

And it's usually due to migration.

GUSTAVO SOLIS (through translator): A study published by researchers at UC San Diego confirms what lifeguards are seeing in the water.

Drownings increased from just one in the four years before the new wall to 33 in the four years after.

The study did not establish causation or attributed drownings directly to the new border wall.

Peter Lindholm is a professor of Emergency Medicine at UC San Diego.

PETER LINDHOLM, Professor, UC San Diego: Well, I think it's important for the lifeguards, the emergency response systems and the UC Health System to know if we have a lot of drowning related accidents coming in.

It's something that the healthcare system needs to help.

GUSTAVO SOLIS: Lindholm co-authored a study with Ana Lussier, a PhD and medical student in San Diego.

She became interested in migrant injuries during the rotation at the trauma surgery unit where she saw a lot of injuries from people who fell from the border wall and wondered whether anyone just tried to swim around instead.

ANNA LUSSIER, Researcher, UC San Diego: Drowning during migration is one of the leading causes of death during migration around the world, especially around the Mediterranean area.

And there hasn't been a lot of scientific investigation into that.

So I'm hopeful that the work that we do can kind of lay a framework for other researchers and other people who care about this issue.

GUSTAVO SOLIS (through translator): Both researchers called this a preliminary study, they would like to analyze more data on drownings and rescues from both sides of the border.

PETER LINDHOLM: I think that we still missing a lot of information there.

And as a scientist, it's interesting to see if we can find information and contribute with that information.

GUSTAVO SOLIS (voice-over): In the meantime, lifeguards expect the drownings to continue and the work is taking a toll.

JASON LINDQUST: It's not just people from Mexico and South America we see Russian, Yemen, we see a lot of people.

GUSTAVO SOLIS (voice-over): Jason Linquist says some stories are difficult to forget, like one migrant who tried to dig a hole under the fence.

JASON LINDQUST: He dug under at a time at lower tide and then he got stuck.

Tide was coming in.

And he was basically underwater every time a wave would come in.

GUSTAVO SOLIS: That incident resulted in a successful rescue.

For PBS News Weekend, I'm Gustavo Solis in San Diego.