For this challenge I used Sensei Xu linked to the Grand Temple as my "flag."
The strategy works like this: On my first turn, I'll reveal Sensei Xu and bring in a Mercenary Helmsman. Then the Grand Temple will go over and touch the Mobilis, which will explore to bring aboard Sensei Xu and the Helmsman, providing it with S+S move, so it can keep up with the Grand Temple. Thereafter, the Mobilis will remain submerged and will tag along directly below the Grand Temple everywhere it goes. The double cancel ability of the Mobilis and Christian Fiore together will keep Sensei Xu safe from capture, and quite possibly will keep the Grand Temple safe from being fired upon first as well.
When either the opponent's gunship or "flag" carrying ship (preferably both in one) comes within range of the Grand Temple's S+S+L attack range or, if I get Shap-'ng Tsai's SAT and the flag is the target, within S+S+S+S+L range, the Grand Temple will attack and attempt to derelict the flag carrier or sink a non-flag carrying gunship. Once the flag-carrier is derelict, I'll explore to capture the flag. If an opponent ship were to get close enough, ramming with both of my vessels is a possibility, but I would not want to risk the Grand Temple being boarded unless I expected a distinct advantage on the boarding roll. If the Grand Temple had to separate from the Mobilis to attack, she will meet up with it again ASAP to have the double canceller protection on the way back home. Just before the final move action to reach home, the Mobilis will either take Fiore aboard or cancel his Ex-Patriot keyword to allow the Grand Temple to dock. If the Grand Temple is badly damaged, I might transfer the flag to the Mobilis to make its way home submerged. I think it can cancel it's own Mercenary keyword to allow it to dock.
The fleet is slow moving, but the opponent has to come to it to have a chance of winning.
I'd have to be sure to steer well clear of reefs to avoid harm from Foul Winds. Favor of the Gods provides Mermaids protection.
I added in 5 nasty UTs to discourage opponents from exploring wild islands to get Periscope or other helpful UTs.
One possible problem would be if the opponent's "flag" was more than 6 points. Then I would have to unload Fiore to load a 9 or 10 point flag or my captain for a 7 or 8 pointer. I could put a 7 or 8 point flag on the Mobilis and crawl home at S while submerged with the Grand Temple running interference.