Boston — Barge operators chugging up and down the nation's 25,000 miles of inland waterways are steering into uncharted waters. The maritime industry faces dramatic changes. Rivers, canals, and lakes carry about 16 percent of the nation's freight. The Department of Transportation forecasts a near-doubling of trade on the waterways by the year 2000, due mostly to a surge in coal and grain exports.
The maritime industry says it can meet the demand. But it loudly complains about difficulties that could crop up if Washington goes ahead with its plan for ''user fees.'' Such fees, supported by President Reagan in his Sept. 24 address to the nation, would be paid by commercial navigation to finance waterway maintenance and contruction - costs mostly picked up by Uncle Sam so far.
One area ready for more traffic is the Great Lakes and St. Lawrence Seaway. ''The lakes are underutilized. . . . The coal market is very new and we are ready to handle it,'' says John Fish, executive director of the Great Lakes Commission, an organization of legislative and trade representatives from the eight states adjoining the big lakes.
Mr. Fish maintains: ''Few people realize that Ohio ports are closer to northern Europe than Norfolk (Va.), or Baltimore (major exporting ports) . . . or that railroads bring us coal from the Midwest, West, or the Appalachian region over flat land. This costs much less than hauling over mountains to reach the East Coast.''
Another plus for the lakes: ''We've been self-unloading coal for 50 years (the boats themselves have the equipment to unload 5,000 to 10,000 tons an hour) , much longer than other areas. We can transfer our loads quickly to deepwater vessels at Montreal and Seven Islands for shipment overseas.''
If the Great Lakes have to increase awareness of their advantages among potential customers, the Midwestern waterways (the Mississippi River and its tributaries) have the opposite task. They have more business than the system can handle.
Around autumn, when farmers are hustling to get their harvests to market, barges at the Mississippi's Lock and Dam 26 face a costly three- to four-day wait before passing through the lock.
Fortunately for the bargemen, this famous bottleneck at Alton, Ill., will be eased by another lock and dam about two miles downriver. The new lock will be able to handle 86 million tons when it's completed in 1987. Last year the old lock carried 73 million tons, with an average wait of 20 hours per tow. The old lock and dam will be largely removed when the new setup is ready. Bill Sutton, the Army Corps of Engineers project manager for Lock and Dam 26, says prospects look good for yet another new lock on the new dam. The dam is needed to maintain a channel depth of some nine feet in the river.
Hoping to relieve the crowding on Mississippi waters, the Corps of Engineers is also constructing the $1.830 billion Tennessee-Tombigbee Waterway. The project, begun in 1972, is more than 65 percent complete.
The ''Tenn-Tom's'' advantage lies in its route. When the 234-mile waterway is completed in 1986, it will connect the Tennessee River (which branches off from the Ohio River) with the Black Warrior-Tombigbee Waterway, which empties into the Gulf at Mobile, Ala. Instead of leading barges to the crowded port of New Orleans, the Tenn-Tom will ''provide a shortcut for exports heading for the Gulf Intercoastal Waterway,'' says Glenda Smith, an economic analyst for the project.
Somehow, Lock and Dam 26 and the Tenn-Tom have escaped the federal budget-slicing razor. But the administration shaved the $1.7 billion Red River Waterway project from the budget last March. Continuation of the project could depend on whether user-fee income is available. The Red River runs from Shreveport, La., to the Mississippi.
The government hopes user fees will put an end to such indefinite postponing of projects. A Senate bill, S 810, calls for full recovery of costs by the maritime industry for the use of its share of the waterways. A 6-cent-a-gallon fuel tax is already in effect.
The maritime industry is hardly enthusiastic about user fees. The argument goes this way: It's impossible to determine fair shares, since waterway construction and upkeep provide many other services - flood control, recreation, hydropower, defense.
In a telephone interview, Transportation Secretary Drew Lewis said the administration will work to recover the costs ''across all modes of transportation.'' He reported progress on Coast Guard user fees, a fuel tax for trucks, and proposed airport taxes.
But what about railroads - the maritime industry's biggest competitor? Amtrak , serving passengers, and Conrail, handling US freight, have benefited from sizable government assistance.
''We're working on that now,'' he said. ''We have not developed a program yet , but there are two areas . . . we'll never be able to handle by user fees: mass transport and Amtrak.''
It's not possible to allocate waterway costs exactly, Mr. Lewis agrees. But ''we somehow have to isolate those charges. . . . No matter what you do, everybody is going to be equally dissatisfied. If everybody's equally dissatisfied, you've probably done a pretty good job of spreading the equity around,'' he chuckles.
Anthony Kucera, president of the American Waterways Operators, isn't chuckling. ''The added costs (of user fees) will only be passed directly to consumers,'' he says. The secretary, however, contends the consumer pays either way - whether it's indirectly through taxes or directly through higher prices.
Mr. Kucera is also concerned about user fees and export flow. ''Countries like Argentina, Brazil, and Australia would love an opportunity to beat us to some of those overseas markets. If our exports cost more or are undependable because of delays, they will.''
Though the maritime industry may be hopping mad over the issue of user fees, Mr. Lewis has one approach he hopes will smooth the ruffled feathers: ''The thing I feel rather strongly about . . . is that you have to phase in user fees over time.'' In the long run, the maritime industry ''will have more control over money, and programs won't be interrupted due to administration starts and stops, budget cuts, and unnecessary regulations,'' he emphasizes.