My co-gm and I are planning on a submersible mission, but it seems like while there are submarines, submersibles, and plenty of aquatic cyberware (big Body Shop fan) it seems that 6e doesn't have underwater combat rules yet? At least I can't remember seeing them and I believe I've at least skimmed all the 6e books.
Anyone have guidance from past editions? We have cash to invest in some one-off toys, it's just a matter of some basic framework and stats.
Status: Wet
done

For real though, 6e doesn't have any underwater combat rules, so you'll have to make up your own. What situation are you trying to do, when you say underwater combat? knife fights in scuba gear, megalodons attacking a submarine, or sub-to-sub torpedos?
I'd recommend keeping things as simple as you can, so I'd first establish an Edge hierarchy. If someone is physically adapted and trained for the sea, like merfolk, they get Edge over anyone who isn't. Someone who is neither of those things will lose edge to everyone, and someone who is one of those things is in the middle.
Create a Knowledge skill for underwater combat that represents training, and give your players opportunity to learn it.
If a character is untrained in underwater combat, they lose the ability to gain and spend Edge.
Aside from that, what rules are needed?