SeaMonkey is a different code base in the sense that the point of divergence was long, long ago. Waterfox on the other hand is very close of recent Firefox versions. SeaMonkey is descended from the Mozilla Suite, from which also Firefox descends, but SeaMonkey retains components that Firefox didn't like email, Composer, IRC chat etc.

BTW, I hope SeaMonkey gets a few more devs and love... from what I can see it's struggling badly. I hope to be wrong on this...