Reports of high surf yesterday brought hundreds of spectators to O'ahu's North Shore, but northerly winds caused choppy waves and less-than-spectacular sights.

Conditions should improve today as the winds switch to the more normal trade-wind pattern, with waves expected around 15 to 20 feet in the morning but decreasing throughout the day, according to the National Weather Service.

Last night around 10 p.m., waves on the North Shore were already at 15 to 20 feet, said Wes Browning, a forecaster with the weather service. Buoy measurements off Kaua'i indicated that the swell had already begun declining, he said.

Though high-surf warnings remained in effect, no surf-related problems had been reported to fire or police.

"It will still be pretty large (today), but we're expecting it to fall below the warning level, 25 feet," Derek Wroe, also a forecaster with the weather service, said earlier yesterday. "At the same time, the winds will also be backing down ... so as far as surfing goes, it's going to be a better opportunity for that during the day."

Lifeguards and emergency personnel warned surf watchers to take precautions and to go only to beaches where lifeguards were stationed, in case of an emergency. There are five lifeguard stations on the North Shore — at Hale'iwa, Sunset Beach, Ehukai Beach, Rock Piles and Waimea Beach — said lifeguard Lt. Pat Kelly.

Kelly said he treated three people — all from one family — for minor injuries after they were tossed by a wave onto rocks at Shark's Cove.

"They wanted to look at the big waves and they got too close," said Kelly.

There are no lifeguards at Shark's Cove, and they were lucky to be able to walk away, said Bryan Cheplic, spokesman for Emergency Medical Services.

"Thankfully they didn't get swept into the water," Cheplic said. "They got swept across the rocks and they got all banged up, scratched up and bleeding."

On Kaua'i, swimmers were urged to avoid the shoreline from Hanalei Pavilion to Ki'i Beach. At 1:25 p.m., the National Weather Service measured 20- to 25-foot sets in Ha'ena and 15- to 25-foot sets in Hanalei.

Ocean Safety officials urged those wanting to go swimming or snorkeling yesterday to go to other guarded beaches around Kaua'i. Novice swimmers and surfers were asked to avoid going on the wet sand or in the water on the island's north shore.

On O'ahu, lifeguards warned hundreds of people about the dangers of going into the water, but there wasn't anything they could do to protect people at unguarded beaches, Cheplic said.

"It's frustrating because places like Three Tables, Shark's Cove and a couple of other places where we don't have lifeguards, people still go to," Cheplic said.

Reach Eloise Aguiar at eaguiar@honoluluadvertiser.com.