A desperate search was under way Monday for three missing fishermen off Hawaii after rescue crews found their overturned boat about 25 miles offshore.

The Coast Guard identified the missing men as Jensen Loo, Clinton Oshima and boat owner Derek Tomas. All three are 30 years old.

Crews were forced to perform a separate rescue when a 30-foot boat towing the missing men's vessel sank. A helicopter crew plucked three boaters from the ocean Monday night, and a fourth man swam to shore. One of the four boaters was treated for hypothermia, but there were no other injuries.

Petty Officer 2nd Class Tara Molle said a helicopter, airplane and a patrol boat were looking for the fishermen. The Navy and Honolulu firefighters were helping the Coast Guard with search efforts. Crews reported 15 to 25 mph winds but good visibility in the search area.

Molle said the group left Haleiwa's boat harbor Sunday morning. They were supposed to return by 5 p.m. but didn't.

Friends and relatives told Hawaii News Now Monday that all three men were avid fishermen and knew the waters well.

"It's kind of surprising…but it's the ocean. I just hope they just find them and everything will be OK," said fellow fisherman and friend Justin Wong.

Crews found their 20-foot vessel overturned about 25 miles off Haleiwa on Oahu's North Shore.

"The Coast Guard did send a rescue swimmer down to confirm if there was anyone in the vessel taking shelter. There was no one found," Coast Guard Maritime Enforcement Specialist Jeremy Takesue told Hawaii News Now.

Takesue said the Coast Guard is searching out to 40 miles offshore.

"Right now we do have strong trade winds. The seas are a little rough especially out that far. Right now the visibility is great so we're doing everything we can out there," Takesue said.

The Coast Guard has urged anyone with information that could help find the three missing men to contact the Sector Honolulu Command Center at 808-842-2600.

The Associated Press contributed to this report.