Hi,
First of all, I would like to apologize for my bad English (I'm French and it's my first post on TC ÂÂ

).
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I'm very interested in this idea of exploring the aquatic side of the 40k universe. It's one of my wish to make a fan-rule for aquatic battles. But I have an other vision of them in the 41th millennium that the sea surface battles.
I think that those encounters are 90% submarine (because classical ships are quite exposed to space-ship and aircraft when they are at the sea surface as it had been previously said).
An other point is that abyssal surface is very big in comparison to ground surface (don't have the values sorry), that's why I think that BFG escorts could perfectly act as submarriners, exploring the sea world, attacking abyssal stations and lurking around sea-side cities.
In fact, this kind of battles could be represented as skirmish between 1 to 4 ship on each side, using similar rules than BFG (mainly for movment, torpedoes and submarines modules) with extended damage rules and the aeronautica's altitude system.
I think that it could be quite easily done, don't you?