Despite misgivings about his ability to turn around a federal watchdog agency that was described as "shaky, rocky and floundering," the Senate Agriculture Committee approved the nomination yesterday of James M. Stone as chairman of the Commodity Future Trading Commission.

Stone, 32, the former Massachusetts insurance commissioner, was approved with only two dissenting votes after two hours of occasionally hostile questioning.

But committee Chairman Herman Talmadge (D-Ga) indicated Stone's nomination will not go to the full Senate for final approval until the committee has confirmed the reappointment of FCTC member Read P. Dunn Jr.

And Sen. Jesse Helms (R-N.C.) said he and others on the committee may try to hold Stone's nomination hostage until the White House reappoints acting CFTC Chairman Gary Seavers.

"I have been given as much assurance as can properly be given that Dr. Seavers will be reappointed," said Helms, noting that eight committee members-both Democrats and Republicans-have written President Carter urging that Seavers be retained after his term expires later this month.

Seavers, a registered Republican who is not politicaly active, holds a Ph.D. in agricultural ecomomics and has run the agency since William T.Bagley resigned as chairman last November. Dunn, a politically-well-conected Democratic cotton man, has served on the agency since it was founded.

Committee members said the expertise of Seavers and Dunn will balance the inexperience of Stone, who acknowleged yesterday "that I have not worked in the futures industry nor have I ever bought or sold a futures contract."

Stone said that since he was nominated by the White House, he has spent three days on an Iowa pig farm, learning about agricultural commodities.

Stone said his education as a financial economist-he has in Ph.D. from Haryard-and his four years of experience regulating the insurance industry in Massachusetts "can be of substantial value to the commission.

"Direct experience in commodities may not be as helpful in developing regulatory management skills as time spent running a major state regulatory agency," added Stone. Later, he acknowleged, "I will have to develop a far more detailed knowledge of agriculture than I now possess."

Stone was backed into a corner by a series of questions from Sen. John Melcher (D-Mont) who later voted "present" on the nomination. Sen. Rudy Boschwitz (R-Minn), who did not attend the confirmation hearing, sent a negative vote by proxy.

"How are you going to control price manipulation?" Melcher demanded, citing recent attempts to corner the futures market in wheat. Stone's responses produced only grimaces from the Montana senator.

Stone told the committee all his assets will be placed in a blind trust during his tenure on the CFTC. He said stock he owns in three firms involved in the commodity business was sold the day before the hearing.