SAN DIEGO (CNS) - Heavy surf and dangerous conditions for surfers, swimmers and boaters will prevail Thursday at San Diego-area beaches amid a high surf advisory.

The National Weather Service issued a high surf advisory as well as a small craft advisory, with both set to expire at 10 p.m. Thursday.

Waves ranging from 6 to 12 feet are possible in some areas, with the highest sets anticipated south of Del Mar, forecasters said.

Among the dangers posed by the rough seas are strong rip currents, minor coastal flooding and coastal erosion, according to the NWS.

Forecasters warned that entrances to harbors -- particularly Mission Bay -- will be difficult to navigate due to breakers large enough to capsize boats.

San Diego Fire-Rescue Department lifeguards announced Wednesday morning that the Ocean Beach Pier will be closed for the duration of the high surf advisory.

Lifeguards rescued a man from the waters near Ocean Beach around 2 p.m. Wednesday after he went in to retrieve his dog, SDFRD spokesman Jose Ysea said. The man, a visitor from Nevada believed to be in his mid-50s to early 60s, was found unconscious and not breathing roughly 75 yards from shore.

RELATED: Man swept out to sea in Ocean Beach as high surf continues.

Lifeguards performed CPR on the man until paramedics arrived and took him to UCSD Medical Center in unknown condition.

The surf was expected to peak late Thursday morning, then slowly subside through theĀ evening, forecasters said.