President Reagan has agreed to give a little media exposure to one of Interior Secretary James G. Watt's pet pieces of legislation, a bill designed to increase waterfowl populations by restricting development on the nation's wetlands.

The announcement reportedly is set for 2 p.m. Friday, when Watt and members of a private-sector advisory group, Protect Our Wetlands and Duck Resources (POWDR), will join with Reagan to announce the bill. It's called the POWDR Act, after the advisory group.

Watt has been pushing the proposal, in various draft forms, for months. White House aides said the timing of its announcement is not an attempt to get the president out front on an environmental issue for political reasons.

Environmentalists say that's just as well. Watt's save-the-ducks bill, they contend, is a bit of a turkey.

The Environmental Defense Fund called it an "unimportant bill that is riddled with loopholes and whose enactment could be counterproductive for wetlands by giving the appearance that they have been safeguarded."

In an effort to get the approval of his advisory group and other administration officials, Watt apparently has removed most of the muscle from the bill. The so-called "prairie potholes" of the upper Midwest, an important breeding area for waterfowl, are exempted from the bill. So are wetlands involved in Army Corps of Engineers projects and oil and gas exploration.

The draft doesn't even attempt to define a "wetland," leaving that to the regulators, and the bill doesn't involve any money other than what would be raised by new user fees for wildlife refuges and a doubling of the cost of a federal migratory bird stamp.

"All he's doing for the hunter is giving them a $15 duck stamp--and this is a pro-hunting bill," said one congressional aide who has seen a draft.

One member of Watt's POWDR group, Rep. John B. Breaux (D-La.), told Interior officials in a House hearing last month that, unless Watt was willing to put some money into wetland acquisition, "POWDR is going to go poof."

Meanwhile, getting its own ducks in a row now that the legislation has been drafted, Interior has agreed to constitute Watt's POWDR advisers as a formal advisory group under the Federal Advisory Committees Act. The Environmental Defense Fund has agitated for months to get that done, and Monday Interior sent the paperwork over to the General Services Administration.