There are boats in 40k, but since spaceships and planes tend to trump naval transports in most circumstances they rarely matter. They feature a bit in the RPGs and background material, but usually are only used by locals for various normal matters, like hunting giant tentacled sea monsters, or dredging acid seas for salvage.
One of the downsides to a navy in a space age setting is that the ocean your ships are floating on is much more easy to bombard from orbit without collateral damage than a Hive City's garrison is.
While true, they can also probably afford to dump everything into defenses, including void generators, and might be able to submerge. Burning through the oceans causes it's own collateral damage.
Plus, they aren't much more vulnerable than a supply convoy on land is, if you're needing to move between cities.
I imagine Void shields don't help too much against tidal waves caused by shells intended to kill spaceships crashing into the sea just outside the shield with the force of a meteor. Which is another problem for ships actually, unless they're very strong even a fairly distant miss from orbit could cause major damage from the amount of water turbulence it causes. The size of most Hive Cities means that a tidal wave is unlikely to damage anything important in them, mostly just drowning/crushing the lower hives and underhive.
It's also easier to mount huuuuuuuge guns on dry land in fortresses and cities than it is on a boat. If you need laser weapons big enough to shoot frigate class spacecraft there's way more room for them and their generators on land than in a ship that needs to move around.
It's also in theory easier to hide a land convoy in rugged terrain or stick near targets that can't be put at risk than it is at sea. Broken ground, mountainous terrain, forests and so on can make it very hard to get good targeting info on a convoy, but the sea is fairly uneventful, if large, and a boat is probably possible to detect with enough scanning.
There are boats in 40k, but since spaceships and planes tend to trump naval transports in most circumstances they rarely matter. They feature a bit in the RPGs and background material, but usually are only used by locals for various normal matters, like hunting giant tentacled sea monsters, or dredging acid seas for salvage.
One of the downsides to a navy in a space age setting is that the ocean your ships are floating on is much more easy to bombard from orbit without collateral damage than a Hive City's garrison is.
Yeah, there are horse regiments in 40k too. Badass but usually not a big deal in serious fights
They mostly serve as scouts and occasional linebreakers supplementing other regiments as I understand it.