There is now provided a rigid closed cell polyisocyanate based foam for use as a positive flotation material for watercraft which meets the U.S. Coast Guard immersion test (CGD 75-168, 33 C.F.R. .sctn.183.114). As a blowing agent for this foam, a mixture of 1,1,1,2-tetrafluoroethane (R-134a) as a frothing agent and water is used, where the amount of water is less than 2.5 weight percent based on the weight of the formulated polyol composition. Also, in preferable embodiments, the polyols in the polyol composition are initiated by at least one amine; and the polyol composition contains a delayed action tertiary amine catalyst. The foams used in the invention have excellent dimensional stability. In another embodiment, there is also provided a formulated polyol composition of R-134a and a polyol composition having an average OH# of less than 400 and an average functionality of greater than 4 using R-134a and less than 2.5 wt. % of water, where the isocyanate and the formulated polyol composition are reacted in a weight ratio of 0.9:1 to 13:1, to provide a dimensionally stable foam for use as a flotation foam in watercraft.