The bell, to use one of Sen. Mike Gravel's favorite metaphore, was about to ring for the 15th round.

"The whole Senate's getting tired of this waterway fight," Alaska Democrat said impatiently. "It's time to slug it out and get a decision. These guys have been dancing in their corners long enough."

In some ways, the prize fight metaphor was a good one for the Senate's extended battle over S. 790, the waterway user charge bill. After months of preliminary sparring, there was a general feeling that the time had come for the main event - a vote on the Senate floor.

There was only one problem with the boxing analogy: the two main fighters were still reluctant to enter the ring.

Although their colleagues were pushing for a final resolution, neither Sen. Pete Domenici (R-N.M.), the sponsor of the waterway bill, which would require barge lines to pay the government for their use of federally maintained waterways, no Sen. Russell B. Long (D-La.), the Bill's leading opponent, was quite ready at the start of this week to take the issue to the floor.

Neither man was sure he had the votes to prevail.

Domenici had won a major victory last June when the Senate voted for a relatively high user charge on the waterways. It was the first time in history that either house had approved a waterway toll, making the vote a considerable coup for a first-term Republican like Demenici.

But the House had subsequently passed much lower fee, and the whole question was back in the Senate.

All this spring, while the full Senate was debating the Panama Canal treaties, Long and Domenici, with assorted kibitizers from the executive branch and various lobbying groups, were holding their own informal debates on domestic canals.

In light of the House vote, Demenici had to admit that he would have trouble getting the Senate to vote again for the heavy toll it approved last June. And Long had to admit that he would have to settle, after all, for some level of waterway charge. So both were in mood to compromise.

But two problems stood in their way.

First, there was a basic difference of principle. Domenici wanted the waterway charge to be set at some percentage of the government's annual expenditure on the waterways; that way, he figured, the bargemen might be less inclined to push for expensive new federal water projects. But long was opposed to any linkage between the government's costs and the tolls it could charge.

At one point, the two men had seemed to reach a settlement on that issue, but then the second problem became manifest: the kibitzers.Both Long and Domenici were representing multi-faceted constituencies on the waterway issue, and these groups' in-fighting made it next to impossible for the senators to settle anything.

Domenici had the support of the Carter administration, but the administration seemed to be of several minds. Lobbyists from the Department of Transportation were urging him to accept almost anything that Long offered, while the White House domestic issues staff told him to hold out for more.

Long was confronted with a constantly changing roster of lobbyists representing separate factions of the waterway freight industry. Some were willing to compromose with Domenici; others told Long he should not give an inch.

When it became clear that he would not reach common ground with Long, Domenici sought out Sen. Adlai Stevenson (D-Ill.), who has held to a middle ground in the barge fee debate. By midweek, though, there was still no sign of a breakthrough.

This made Gravel and numerous other senators indignant. The user charge legislation had been attached to an omnibus water resources authorization bill that included federal projects in 34 states. With elections coming up, the senators wanted the bill passed so they could go home and crow about those projects.

Gravel and his colleagues started pushing the majority leader, Robert C. Byrd (D-W.Va.), to put the waterway vote on the calendar. Byrd was anxious to dispose of it, too - the bill includes a big flood control project in his home state - but as the custodian of the Senate's work schedule, Byrd had another worry: if the bill came up, Long and Domenici could debate it until doomsday.

In the House, there would have been so such problem, because most bills are assigned a specific period for debate before they reach the House floor. But Seanate rules provide for unlimited debate; without an agreement between Long and Domenici, the waterway debate might, indeed go on without limit.