So what is there in the Warhammer 40,000 mythology to work from?
Well the Imperial Guard are the army, and apart from the other human military organisations too numerous to mention here, the Guard are the closest thing to the Navy. In fact, the boys at Forge World and Games Workshop have actual begun to release Imperial Navy models and a handful of infantry figures (a pilot, some crewmen and an Officer). So far most of these are merely amalgamated into Imperial Guard armies.
I have been inspired by the obscure and fleeting references, mainly in several Games Workshop sci-fi books that run parallel to the minatures, of Naval soldiers. This was the spark that gave me a loophole to project my entire run of ideas.
WH40K refers to Armsmen which are employed on Naval ships as guards, enforcers and 'in modern parlance, marines' used in boarding actions (not to be confused with human Space Marines, also a WH40K product). Apparently they are highly trained, in comparison to Imperial Guardsmen, use a shotgun-like ballistic weapon and some form of armour similar to the Imperial Guard special forces the Stormtroopers.
The new Navy models are stunning and already very popular with Imperial Guard players. Reffered to as 'flyers' these massive warmachines can 'swoop' in across the tabletop and scare the pants off the opposing player.
So popular have the Imperial Navy models become that infantry seemed natural to use with them and so the Imperial Guard droptroops and other figures have now been incorporated into the Imperial Guard product series, but still no sign of Armsmen or fighting naval infantry.
Naval figures have been limited to the 'Regimental Advisor' which represents Imperial Guard contact with 'eyes in the sky' orbital ships, as well as pilots. So these are all I had to go on to develop my own ideas.