I don't really think a beach assault is necessary in 40k. The use of sea based navies is quite rare (plus, a lot of planets wouldn't really allow it, even if there was any point at all). I love the idea of assaults, but this one misses the mark imo. There is naval warfare in 40k. It's just barely ever mentioned outside of the odd novel, or the rare codex story (I vaguely recall a Space Wolf story where they invaded a domed, underwater Tau city). The Orks even had a rule set for a submarine back in one of the editions of Apocalypse.
To be honest, I don't see why the factions would use naval transports and the like if they have plenty of aircraft to bus their people around. It's significantly faster, allows for higher mobility (in the sense that aircraft is not limited to the water only), and probably safer unless the entire planet is one big flak emplacement. That being said, seeing Orks charging a coastal battery with attached bunker a la Wolfenstein:ET would certainly be nice. Especially once there are more Orkz on the beach than there is sand.
Um your wrong . There is naval in 40 k . Orkz actually have boats and submarines . There was a white dwarf magazine about a battle where space marines defended a port from orkz who came by boats and subs.
http://forums.relicnews.com/showthread.php?253990-warhammer-40k-at-sea . Proof right here for you to read. So it's is possible to have battles on water and lakes using boats. Also they say tau have dome cities under water. Depends if be ever want to do naval sea battles or not one day.
Thanks nazgrub perfect picture. Also they also have records of space wolves fighting tau underwater too.