Multiplayer only worked at EnigmaCon, we never made it further than that. The engine was Intrinsic - a licensed engine. Don't know if it's still around.
Mods - we created a bunch of tools for modding that I shared a couple years ago - here's the link to my drive folder:
https://drive.google.com/folderview?...&usp=drive_web
It's not simple to mod, the code for modding - and even encrypted competitive offline scorekeeping is there.
I still mess around with Deep Six, but there are so many very cool sub games, I doubt I'll ever do more than drive around in it and enjoy the untextured (but colored!) land masses and un-simlike water.