THEY were a 1960's counterculture fad at first. Along with ripped, faded bluejeans, bandannas and long hair, water beds were yet another way to make the statement: We're against convention. ''Really, the water bed is about the only symbol of that era that's lingered,'' said Jerry Long, director of sales and marketing at Wavecrest, a California-based waterbed manufacturer.

Water beds have not only lingered - they have thrived. Over the last decade, water bed sales have increased by about 100,000 units each year.

Last year, 3.4 million water beds were sold in the United States. When the prices of accessories -chemicals for the water, special linens and the like - are counted in, water beds represented a $2 billion industry in 1984, according to Facts Consolidated, a Los Angeles research firm.

Conventional bedding manufacturers are jumping into the market, either through acquisitions or additions to existing product lines. In the last two years, the Ohio Mattress Manufacturing Company, a large manufacturer of conventional mattresses, has acquired Wavecrest and Monterey Manufacturing, as well as Wood Stuff, which makes waterbed frames. The three acquisitions are generating more than $60 million in annual sales for their new parent.