I dont know what that much water sloshing about topside would do to the ships trim. Lets see...a 5" naval gun tube & recoiling acceorys weighs how much? And water weighs in at one gram per cubic cm. So, how many cubic cm do you have to confine in the recoil tank to counter the gun? Also that plentifull water is brine. Highly corrosive. You will need to add in extra corrosion protection to this system to protect the gun & service machinery. Maybe something with hydralic oil & compressed air???

And that's just recoil, you then have to think about how to get run-out (ie get the ordnance back to its firing position). Recoil systems capture energy during recoil and use it for run-out. Of course you could have a team of jolly jacks with ropes, a la Trafalgar, but I don't think it will be popular.

plus you'd also have to move this water around to match the guns elevation and traverse, contain it so it's not in the way of reloading and find a way to mitigate the initial shock of impact on the water. In no time at all, you're back to the hydro-pneumatic recuperators (I think that's the correct term) that are already used.

You wouldnt have to move the water around, just have it a large semi-circular area, so it can take the breach block at any elevation. Have it a pressurised pool, to better handle the recoil, and have the water flow out of pipes rather than simply over the deck, to be pumped back in rapidly to re-pressurise the container and push the breach closed... But yeah, I wasn't really expecting the idea to work. :D

Might make serving the breach a bit tricky. Pressurising an open pool would be an impressive trick, too. Oh, and having bodies of open water on a ship does no good for its stability.

No no, I never meant for it to be an open pool, but an enclosed tank around the gun.

So how do you put the shells in? Perhaps the ideal would be that the two pools were enclosed in a cylinder with a tight fitting piston, and also included some air to stop the mechanism hitting the water too hard.

Meh, its belt fed. :D Like I said... not an engineer. I can hold my water (so to speak) concerning small arms mechanisms, and thats about it.

A belt-fed artillery piece? I met a girl like that once...

Ok lets assume a 20 ton tube, including the accesorys attached that add to the recoil mass. That translates to about 18000 kilograms. Which is 18 cubic meters of water if I've done the math right. Add in the bulk of the tanks and their mass, set all that on a rotating station and watch the engineeering dept go nuts keeping the ship in trim. To accomplish that task they will need a system of tanks below deck, near the keel perhaps, to pump water back & forth. This has to be done at a fairly high speed. So now the ships engineering plant just got another layer of complexity. And the ship got bulkier below the water line, and the crew got fatter as well to handle the additional battle stations tasks and maintiance. Looks like were are still gonna use the hyropnuematics...