Hm, you could also give them an increased focus on skimmers and flyers to replace their lack of assault squads, since they'd be able to cross water with ease. Though you'd require a good forge world nearby because anti-gravity tech is supposedly "rare".
I hadn't thought of that but it could be solved by something else. In their home system I planned on having an industrial world that is also a Shrine World controlled by the Ecclesiarchy. Seeing as how the Adeptus Mechanicus doesn't have total control over the Imperium's technology (but obviously it has a lot, hence the fact they're the primary supplier of the Imperium's weapons and vehicles) I figured that said planet devotes a portion of its automated manufacturing facilities to making most of the chapter's arms and some vehicles in exchange for protection from whatever hostile forces wander into the system and pose a threat to what relatively sparse population there is on the planet. Both parties obviously benefit massively from such a deal. This would be one of the main reasons behind their good relationship between each other.
So the Heralds' homeword is called Kyrion I and, as previously stated, it is Death World covered with vast oceans and only has a few small islands on which the human population lives. Unlike the Space Marines Kyrion I's regular human inhabitants deal with the monstrous aquatic wildlife the old fashioned way, small boats and a lot of harpoons and the occasional stub revolver for the smaller creatures. The humble stub revolver has been the weapon of choice for the vast majority of the planet's population due to its reliability and ease of use (and for one notable member of the chapter the preference for revolvers very much stayed with him, but we'll get to him later). Kyrion II used to be a dead world which the Heralds would often use a training ground for combat in the most hostile environments possible due to its extremely toxic atmosphere and high volcanic activity (the toxic atmosphere being caused by said activity releasing high amounts of sulphur into the air.) so the only human activity is a small outpost manned by about half a dozen Chapter Serfs. Kyrion III is an agri world and Kyrion IV is an industrial world that is also a Shrine world run by the Ecclesiarchy due to it being the site of a large battle during the Cataclysm of Souls. Kyrion IV is important because the Ecclesiarchy supplies the chapter with weapons and vehicles from the planet's factories in exchange for protection.
