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...

When the decision was made to make a browser-only version of the Mozilla suite, a separate front-end was built on Gecko eventually called Firefox. The legacy suite was renamed SeaMonkey, and it still uses the same repository. The trunk version of SeaMonkey uses the trunk version of Gecko, and it doesn't fork or modify the stock Gecko in anyway.

Seamonkey is much older, and not really a fork as it follows current day Firefox development quite closely.

Hell, one may well call Firefox a fork of the browser part of Seamonkey.

A more interesting comparison to Waterfox would be Pale Moon. And there is also Basilisk, a more recent project by the Pale Moon people.

Sadly, Pale Moon is still not available on Mac. I wasn't aware of Basilisk, though! It appears that that one too is Windows/Linux only.

I thought SeaMonkey was still the entire suite (email client, HTML editor, etc). It seems to be further behind in keeping up with Firefox.