Actually, the gill man survives in the first two movies. This is evidenced by the fact that even though the last scene shows him sinking, apparently dead, it is never made absolutely clear he is. Also, the fact he is alive in the same lagoon in "Revenge of the Creature" and brought back to civilization, shows he survived the injuries from the first film. No dialog indicates that he is not the same creature, or that there was more than one Gill man. Same as with the ending of "Revenge of the Creature", where the last scene is of him sinking again, this time in the waters of Florida. When "The Creature walks among us" begins, the creature is still in Florida waters when he is captured again. This proves that in the first two films, the Gill man actually survives in the end. In all truth, it can be assumed that he doesn't not survive in the end of "Walks among us", because it ends with the creature entering the ocean, trying to return to the world he knew, but because he no longer has his gills at this point, the audience can draw the conclusion that he ended up going to his death by drowning.