Ao no 6-gou

Range Murata is the man of the project, based on his draws and concept art, the two animes follow the same aspect but a little diferent storyline, one in the future, and the other in a near future. 

As others have pointed out: Studio Gonzo (Meaning a certain early-adoption of digital animation and CGI) and character designs by Murata Range (Therefore a short-haired tomboy). Both deal with environmentalist themes, military crews, a jaded older character, stories centered around a device that will change the fate of the world as well as slightly confusing endings. 

Sci fi series centered around a big ship and its crew, while the male and female leads in both shows specialize in in piloting a smaller "fighter" craft. Last Exile takes place in the sky while Blue Sub 6 takes place in the sea. Both shows were made by Gonzo and heavily feature very early and rudimentary 3D CGI. 

Both take place on a world submerged underwater with people fighting for supremacy with an alien race. They have many similar ethics and main characters. 

On the surface, these anime are similar since they are about submariners fighting for humanity against lifeforms that rose from the sea, and they both provide the thrill of naval warfare with plenty of excellently-animated submarine scenes. But at a deeper level they are really about the enemies, born into the world with intelligence rivalling that of humans, as they struggle to understand their newfound emotions and find a moral way forward. Aoki Hagane was surely inspired by Ao no 6-gou, but offers its own take on these themes.