I examined both of the save files and confirm the bug: Automated work boats will sometimes not build the appropriate improvement. This includes both fishing boats and offshore platforms. In the Darius I 830 save, I could cause the WB to build a FB automatically by moving the WB around manually and then automating it. Sometimes this worked and other times it didn't. It's also odd that when you select the automated WB, you have not only the option to Stop Automation, but also the option to Build Improvements (Automated). The Wu Zetian 1991 save is similar, with the automated WB not building an offshore platform.
I'll move this thread to the Confirmed Bugs forum and add it to the Bug List.
Moderator Action: Moved to the Confirmed Bugs forum.