SAN FRANCISCO -- In San Francisco Bay, it's been a great week for whale watching thanks to some special visitors seldom seen in those waters.

It is an unusual sight to be sure: Humpback whales frolicking in the shadow of the Golden Gate Bridge.

"It's pretty dramatic, and it's delightful to have wildlife of this grandeur in San Francisco Bay," Jeff Boehm, executive director of the Bay Area's Marine Mammal Center, told CBS News.

Boehm said the migrating whales are following food into the shallower waters.

"The problem occurs when our zeal to get close to them interferes with their natural behavior," he said.

Terry Parks shot video while kitesurfing near two of the whales.

Marine biologists say humpback whales have been swimming into San Francisco Bay in unprecedented numbers during the past two weeks. KPIX

"There were a couple of times there where you can't see where they're gonna submerge, and then they come up pretty close to you," Parks said.

Over the past few days, boaters have seen whales breaching in the bay.

And while the 60,000-pound whales are considered docile, Boehm pointed out, "All of these animals are going to be unpredictable, and these humpback whales can be rather aerobatic as well."

And that can be dangerous -- as kayakers in Monterey Bay found out last fall.

Another concern for scientists is that the whales could continue to swim inland, up the Sacramento River and get stuck as a humpback named Humphrey did back in the 1980s and '90s.

"A tide can shift relatively quickly, leave the animals in water that's not navigable," Boehm said. "Also, as they move up into rivers, into fresh water, which can start over time to have effects on their skin."

Marine biologists say warmer ocean temperatures caused by El Nino are drawing in marine life that would typically only be seen in deeper waters.

How long the humpbacks will stay in the bay -- and whether it's part of a larger pattern -- is still unclear.