With its opponents perched somewhat unhappily on board; a watered-down waterway toll bill glided past another milestone yesterday in its journey toward a final House vote.

At the urging of Rep. Al Ullman (D-Ore.), chairman of the Ways and Means Committee, and Ray Roberts (D-Tex.), ranking member of the Public Works Committee, the Rules Committee voted yesterday to pass on to the House floor the waterway bill, which would force commercial barge lines to pay for their use of the nation's inland waterways.

It was not the happiest task Ullman and Roberts ever shouldered. Both are strong opponents of the waterway fee. Indeed, Ullman told the Rules Committee that the Senate-passed waterway bill, S.790, "would totally destroy the whole transportation system."

But Ullman and Roberts are realists. They think that some form of waterway fee is likely to pass Congress this year. So their two committees had set about to devise the smallest fee they could get away with.

They settled on a federal tax of 6 cents per gallon on the diesel fuel that drives the barges. Even that figure was anathema to the barge industry, but it is far lighter than the fee the Senate bill would impose, and had won the grudging support of most bargemen.

Having been approved by both Ways and Means and Public Works, the waterway bill was ready for the House floor. But to gain admission to the floor, a House bill needs a ticket - in parliamentary jargon, a "rule."

The ticket booth is the Rules Committee. It controls the flow of legislation to the floor by parceling out tickets, or "rules," that establish when a bill can come up, how much debate there will be, what kind of challenges can be made and other conditions.

Like most traditional institutions.

Like most traditional institutions, the Rules Committee has developed parlance and procedures all its own. There are "open rules" - permitting almost any amendment on the House floor - and "closed rules" - permitting no amendments to a committee bill - and various shades of rule in between.

Ullman and Roberts yesterday were asking for a "modified open rule" that fits neatly with their overall view of the waterway fee. The rule they asked for would have permitted floor amendments to cut the fee established in their bill, but would not have allowed any effort to raise it.

Sitting beneath a huge crystal chandelier in the Rules Committee's elegant chamber in the Capitol, Ullman and Roberts presented their case. The two apparently had not had time to do their homework; they disagreed in their answers to some of the committee's question, and on some others they simply ducked.

The Rules Committee was deferential, until Rep. Christopher J. Dodd, a brash, young Connecticut Democrat, gained the floor.

Wouldn't it be a little fairer, Dodd asked gently, if the rule permitted amendments to increase the tax as well as to cut it?

A few members muttered in agreement, but they were cut off by the gruff, stentorian voice of Rules Committee Chairman James J. Delaney (D-N.Y.), who said: "If we open this thing up, we'll never get a bill off the floor."

Ullman and Roberts were followed to the witness stand by two junior congressmen, Berkley W. Bedell (D-Iowa) and Robert W. Edgar (D-Pa.), who asked for a rule permitting a floor vote on an amendment that would establish a waterway fee about twice as high as the Ullman-Roberts plan.

When Bedell and Edgar had finished, the Rules Committee seemed split. Consequently, the members settled on a compromise: the fuel tax bill would go to the floor under a closed rule, permitting only one vote, yes or no, on 6-cent-per-gallon plan.

There would be a chance for both sides to challenge that rule on the floor. But the full House rarely second-guesses the Rules Committee. A House vote on the waterway toll - 6 cents per gallon or nothing - seemed assured.