Would be nice to have them officially included as extras, but it looks like that for some reason it was not possible:

9 maps made by developers :
https://www.fileplanet.com/archive/p-38917/Submarine-Titans-Maps

Official wallpapers pack:
https://www.moddb.com/games/submarine-titans/downloads/submarine-titans-wallpapers

Those can be seen on archived official page:
https://web.archive.org/web/20010405001514/http://www.subtitans.com/

They really should've included the manual too, since many things are not explained in-game.

https://ia600203.us.archive.org/5/items/Submarine_Titans_Manual/Submarine_Titans_Manual.pdf

Thank you for link to this Manual, it's good quality and useful!

I got warnings of malware when trying to download those maps from the first post in the thread and also the link to the archived official page no longer works.