I've done an ACL, twice.
The doc has zero issues with diving, but has issues with rocky shores and rocking boats.
I'm 4 months post surgery, it is getting stronger and more important more stable. I was told at 4 months I can start using it a bit more aggressive, so this weekend I am taking the motorcycle out for a ride. Yes, that is aggressive, observed trials has no seat and you stand the whole time. I'll try water again probably a week later, if I don't go up to Moab to go riding some more.
The advise I got on the knee brace is to use it as little as possible. Only when there is a chance of tearing it up (motorcycle riding, crashing). I have zero plans to dive with it. The docs want the knee to gain strength, not protect it to the point where it doesn't gain strength.
Don't know what king of serious injury you had, nor the advise the knee doc has about it.
I guess I could see a point in using a brace on an un-repaired knee. Both of my braces only fit bare skin or a light sock. There is zero chance of wearing one on the outside of a suit (wet or dry). The only outside option I can think of is the one size fits all brace. But those are nothing compared to a proper fitted brace. The only inside the suit option that would exist would be a drysuit, not going to fit a wetsuit over a brace. I would be worried about wear points tearing up the inside of the suit.