To the real estate editor: Regarding the ''Walking bridge over swamp'' inquiry in the Ask a Builder column, we have for many years maintained a floating dock in salt water here.
The dock was constructed of a wooden frame over eight 55-gallon steel drums. It seems to make no difference in longevity whether the drums are painted, since they rust and corrode from the inside due to condensation. They do survive for at least several years before becoming waterlogged.
Replacement is then an easy matter by ''popping'' them under the framework. The drums have never buckled even though I have seen them embedded in 8 inches of ice.
The used drums are purchased for several dollars each. Suppliers can be located in the Yellow Pages. Robert B. Bosler Donald B. Lehy Woods Hole, Mass.