They also say the industry lacks a proven alternative to copper paint.

Some recall when tin was banned in bottom paint, and copper was hailed as an ideal replacement. They see the current ban as part of a frustrating cycle that could eventually lead to the new, more expensive paints being prohibited in the future.

And many of the marina's most affluent boaters oppose the mandate on principle. Some say they have owned a boat for years without ever having to pay to strip the paint, and they resent being forced to take action that they believe won't solve the marina's copper problem.

Copper is found in car brake pads, tools, coins and other external sources that dump into runoff, and eventually into the various water sources that fill the marina. Copper-filled water from Oxford Retention Basin and Ballona Creek is therefore also raising the concentration of copper in the marina, boaters argue.

Larry Silver, 77, took it upon himself to pay for a bus that will transport dozens of boaters from the Del Rey Yacht Club to the downtown water board meeting Thursday.

Smoking a cigarette in the galley of his 68-foot yacht, Silver questioned the science behind the copper reduction plan and wondered where the county can dump all the toxic sediment if officials decide to dredge.

"As an environmentalist, if they told me, 'Put this type of paint on your boat that costs $5,000 more and helps the environment,' I would be all for it. I raise tropical fish," he said. "But this program is stupid. It's going to do harm. Anything that's alive will be killed."

Sergio Sanudo-Wilhelmy, a geochemist at USC who has studied trace metal concentrations in Southern California coastal waters, said dredging can pose additional water contamination problems because it can release copper that has been trapped under the surface for decades.

Another option regulators are considering involves "capping" the entire bottom of the marina with a new layer of sediment. That would cost an estimated $19 million, significantly less than dredging.

Sanudo-Wilhelmy said science has come a long way in the 30 to 40 years since boaters started putting copper in the marinas, but effective non-toxic paint options remain limited.

"The sediment is polluted, but we don't know who put it there and when," Sanudo-Wilhelmy said.

Sewage treatment plants used to release a lot of copper into coastal waters, he added, but clean water regulations have caused those discharges to plummet over the last 50 years.

The water board's plan now calls for about a 70% cut in the marina's copper concentration level by 2024.

To help accomplish that, they want 85% of the marina's boats to use non-copper paint by the same year. The plan also calls for dredging to be completed by 2029.

L.A. County officials called those compliance dates and targets "unachievable" and "unrealistic."

Like some locals, L.A. County also called for more detailed studies similar to one that was conducted in San Francisco Bay.

In the Bay Area, further studies found the copper from some boats was less harmful than originally believed, leading officials to push for less stringent regulations.

matt.stevens@latimes.com

tony.barboza@latimes.com