Can be seen in the replay when clicking on the unit. The last rework of that "ConquestCritical" entity flag was in #3229. I recall players can still survive with docks (so they could still be able to produce military and trade ships. Docks give population too, so if one has many docks, one could do something with it. Should probably remove it though.)
Looking at the fishing ship template, the "-ConquestCritical" removal doesn't exist there either, so should likely be added there. While at it, might think whether we find some other units that shouldn't stop the player from getting defeated. Someone said he survived a game with a Healers. Same story... He could do something with it, but on the other side, he won't be able to rebuild.