I dont think they would have certainly died, but I wouldnt keep them in saltwater for long if they are figure 8's. Most brackish water fish can survive through some very radical changes in salinities because in nature, the tides and rainfall amounts play apart in how salty the water is. Mollies, for example, can acclimated to almost any type of water from fresh to saltwater and back, so your puffers could probrably handle some time in true saltwater.