THE walls of a recently completed university boathouse here may contain a key answer to the nation's growing garbage crisis. They are made from the ash of incinerated garbage.Here on Long Island the garbage disposal problem is treated with special urgency. Towns on this finger of land that stretches 125 miles east of Manhattan rely exclusively on underground aquifers for drinking water. Accordingly, the state ordered all landfills closed by the end of 1990. Most major Long Island towns now compost, recycle, and incinerate. The ash left from incineration has long been the weak link in waste-to-energy technology. Most environmentalists oppose incineration as dangerously toxic, polluting either the air into which the fine ash flies or the ground in which the bottom ash is buried. United States Environmental Protection Agency (EPA) officials, however, say much of the concern is based more on emotion than fact. "Incinerator emissions are not a major problem," says EPA spokeswoman Robin Woods, noting that advanced pollution-cont rol equipment is available to meet federal clean-air standards. She says several hundred large cities now operate incinerators and several hundred others still plan to build them. Residents of most towns on Long Island have long felt they had little choice in the matter. Most of the 1,200 tons of ash produced here each day is shipped to out-of-state landfills. Spurred by Long Island's need for better answers, the Waste Management Institute here at the Stony Brook campus of the State University of New York has been trying to find imaginative ways to recycle rather than dispose of the ash. Five years ago Institute researchers installed two artificial ash-block fishing reefs in Conscience Bay on Long Island Sound. The results so far according to Frank Roethel, a chemical oceanographer with the university, are promising. Numerous tests show no change in the chemical or structural strength of the ash blocks over time or any release of metals or other contaminants into the tissue of the many marine organisms that feed and spawn on the reefs. It will take at least another two years before university scientists know as much about the behavior of the ash used to build the boathouse. Completed only last spring, the structure used 300 tons of ash or about one-fourth of the island's daily production. Dr. Roethel, who rests his foot on a sample ash block during a chat in his office and invites a visitor to "give it a good kick," says the research will include a wide array of tests. Soil samples will be taken periodically to see if any contaminants from the ash leach into the soil, and the air quality inside the boathouse will be regularly monitored. If any problems occur, the boathouse, which will store small research vessels owned by the university, can be taken down and rebuilt with concrete blocks. However, Roethel says he thinks the tests will show that the ash can be safely recycled. In time, he says, public acceptance will probably be strong enough to allow the use of ash, which acts as a substitute for sand and gravel, in other concrete products such as highway barriers, cesspool rings, and underground vaults. He notes that ash from incinerated garbage was widely used in roads in the East between the 1930s and 1950s before clean-air standards forced a shutdown of most garbage incinerators and c ities turned increasingly to landfills. Roethel says he probably played softball on streets of ash while growing up in Brooklyn. "I wish I could win a Nobel Prize for this, but it's not new - we're only turning a page in history," he says. Environmentalists say that reducing the amount of waste in the first place and recycling as much of it as possible are most important. To tackle the ash question now, says Natural Resources Defense Council lawyer Eric Goldstein, is to approach the garbage problem backwards. "Let's worry about that in 10 years, after we've got the recycling infrastructure up and running," he says. In his view, recycling must be seen as a key component of solid-waste management rather than an add-on. He says the business c ommunity needs the assurance of a steady supply of recycled materials to create a market for them. Environmentalists also are concerned that building incinerators may diminish the incentive to recycle. Roethel disagrees. He says recycling 100 percent of all waste is a laudable but unrealistic goal. The US would lead the world if it could recycle even 50 percent, he says. In the meantime, he sees research into alternate ash uses as crucial. "The garbage is here today," he stresses.