PORTLAND, Maine (AP) — The Lobster Dip was launched 21 years ago as Maine’s first happening where people could raise money for a good cause by stripping down in the freezing winter air and running half-naked into the bone-chilling ocean.

These days, it seems as if everybody’s taking the plunge, and that’s threatening to saturate the fundraising field.

Cold-water charity events have even spread as far as Hawaii, where volunteers last fall plunged into a pool filled with iced-down water to raise money for Special Olympics Hawaii.

Rocky Frenzilli helped organize the original Lobster Dip to benefit Special Olympics Maine and has seen it grow from about 15 participants to more than 300 each New Year’s Day at Old Orchard Beach. Since that first Dip, he has greeted each new year with an icy splash.

Some people call him crazy, but he says it’s his way of giving back to the community.

‘‘As long as I’m physically able to jump in the water I’ll continue doing it,’’ said Frenzilli, 60, a high school teacher. ‘‘It makes you feel real warm inside when you’re doing something special for other people.’’

However, the growing number of such events and the limited number of people crazy enough to jump unprotected into icy water is raising questions about whether these polar plunges are losing their luster and cutting into one another’s fundraising efforts.

These fundraisers in Maine will benefit Special Olympics, a domestic abuse nonprofit, a Ronald McDonald House, an animal welfare group, an environmental organization, an animal shelter and Camp Sunshine, a lakeside camp for children with life-threatening illnesses and their families.

They’re all great causes, said Phil Geelhoed, president and CEO of Special Olympics Maine. But he’s concerned that they might be taking away from the Lobster Dip, which raises about $40,000 for Special Olympics. The figure has been flat in recent years.

Special Olympics chapters now hold more than 40 Polar Plunges each year, raising more than $10 million. One of those dips drew more than 3,500 people who dove into the Atlantic Ocean last February at Virginia Beach, Va., raising more than $850,000.

One of the newcomers, Maine’s Camp Sunshine, held its first Polar Dip in 2006; this winter, it’s holding 10 of them in eight Eastern states, from Maine to Virginia, and even one on the Pacific coast in Seattle.

They’re so successful — they raised $150,000 last winter — that the camp is thinking about taking a page from Hawaii’s playbook and holding one in Florida in an ice-filled swimming pool, said Michael Smith, Camp Sunshine’s director of special events.

‘‘There are so many organizations that use it as fundraising tool, and we’re fortunate to catch it on the upswing,’’ Smith said. ‘‘It’s one of our largest annual fundraising events.’’

The idea of greeting the new year with an icy blast of water isn’t necessarily new. Hardy — some say crazy — groups have been doing it for years, especially polar bear clubs in northern Europe. It’s the idea of cashing in that’s relatively new.

When a certain type of fundraiser is successful, it’s only natural that others will imitate it, but such events typically have a life cycle of waxing and waning, said Michael Nilsen, spokesman for the Association of Fundraising Professionals in Arlington, Va.

For example, the 1990s saw an explosion of fundraiser walkathons, bicycle rides and running events — to the point where they lost some of their novelty.

‘‘When organizations see something that works in their community, they might try to copy it,’’ Nilsen said. ‘‘At some point, they might reach the oversaturation stage.’’

The Natural Resources Council of Maine environmental group is joining the crowd this year, holding its first polar swim fundraiser on Jan. 2 in Portland to help celebrate its 50th anniversary and to raise money for its work on global warming issues.