The Senate last night completed confirmation of all but one of President Reagan's department heads, with Interior Secretary James G. Watt, the target of environmentalists' wrath, compiling the most negative votes.

Still pending, along with the nominations of several Cabinet-level officials, is Raymond J. Donovan, whose confirmation as labor secretary was held up while the FBI investigated charges that he made payoffs to ensure labor peace for his construction firm.

The FBI said Wednesday that it had found no evidence to back up the allegations, and hearings on Donovan's nomination are to resume Tuesday.

Despite nearly four hours of arguments over Watt's record of opposition to many pro-conservation policies of the Interior Department, his confirmation was not in doubt, and he was approved, 83 to 12 with mostly moderate-to-liberal Democrats opposing him.

But some Republicans as well as Democrats expressed reservations. "I have become convinced that James Watt will moderate his views," said Sen. Robert W. Kasten (D-Mass.). Sen. Paul E. Tsongas (D-Mass.) called his vote for Watt a "long-shot risk."

Voting against Watt were William Cohen (R-Maine) and Democrats Joseph Biden (Del.), Dale Bumpers (Ark.), Christopher Dodd (Conn.), Edward M. Kennedy (Mass.), William Proxmire (Wis.), Patrick Leahy (Vt.), Carl Levin (Mich.), Howard Metzenbaum (Ohio), Don Riegle (Mich.), Paul Sarbanes (Md.) and Daniel Patrick Moynihan (N.y.).

Seven other nominees breezed through the Senate as it met in nearly round-the-clock session to give the new president almost all his department heads by the end of his inaugural week.

Approved yesterday were William French Smith, attorney general, 96 to 1; John R. Block, agriculture, 98 to 0; Malcolm Baldridge, commerce, 97 to 1; Samuel R. Pierce Jr., housing and urban development, 98 to 0; Lewis Jr., transportation, 98 to 0; Terrel H. Bell, education, 90 to 2, and James B. Edwards, energy, 93 to 3.

The dissenting vote on both the Smith and Baldridge nominations was cast by Proxmire. Voting with Proxmie against Bell was Sen. Dennis DeConcini (D-Ariz.), and voting with Proxmire against Edwards were Dodd and Kennedy.

Approved earlier in the week were Casper Weinberger, defense; Alexander M. Haig, state; Richard S. Schweiker, health and human services, and Donald T. Regan, treasury. Bill Brock was confirmed for the Cabinet-level post of U.S. trade representative.

Watt, who tangled repeatedly with envirnmentalists as president and chief legal officer of the Mountain States Legal Foundationl, was characterized as a man who would "bring balance to the Department of Interior" by Energy Committee Chairman James A. McClure (R-Idaho). he was defended as an environmentalist, too, by Sen. Malcolm Wallop (R-Wyo.), who said Watt compiled "one of the great conservation records" when he was director of the Bureau of Outdoor Recreation.

But Sen. Kennedy accused him of a "long history of advocacy for special interest against the public interest," and Metzenbaum said Watt's confirmation would herald "a massive shift away from conserving our natural resources."

Bumpers accused the foundation that Watt headed of acting in the interests of its major contributors and called him a "developer at any price." There is, said Bumpers, "at least a very grave appearance of conflict of interest."

Smith also drew some Democratic fire, although Assistant Minority Leader Alan Cranston called his fellow Californian "one of the [state's] best-known and highly respected lawyers." Biden dissented, saying, "I've never found anyone with fewer answers to more questions than this nominee. He's almost totally devoid of an opinion on anything at all. I find it incredible he is moving into a job with so few opinions in so many areas."