No, that will not do anything for recoil. The harder the gun shoots then the more recoil you get. Increasing the mass of the gun helps reduce recoil, or more accurately the mass being pushed rearwards, and that includes you as the diver if you were rigidly coupled to your gun. What we see as recoil is the relative movement of the gun with respect to ourselves as we are only loosely coupled to the gun unless the opportunity can be taken to brace up for the shot with both hands firmly on the gun.
The whole idea of the Mamba system is to allow the gun to be loaded while it is underwater. Some water going down the barrel is good, in fact the gun relies on it to stop dry friction on the internal barrel wall tearing up the front piston seal. Fortunately there is always some water in the front end of the gun after the first underwater shot and that gets smeared on the inner barrel wall during the next shaft reload. I dismantled a gun where shooting it dry (on land I suspect) had turned the "O" ring seal virtually inside out, it was some clear stuff rather than the usual black rubber. The gun only had the one seal on the piston, so must have blown all its compressed air.
One way to let a controlled amount of water in on a Mamba modified gun is to delay the Mamba slide corking up the muzzle as you push the spear in. If the shaft pushes the piston in for a centimetre or so before the muzzle seal takes up then a set amount of water will enter the barrel until the muzzle seal stops any more from entering. This water may decrease the thump of the piston on the muzzle shock absorber, however let too much water in and it will slow the shaft. Ideally you want the shaft to jerk free of the piston while travelling at maximum speed and then decelerate the piston. I got rid of this piston impact noise by switching to a hydropneumatic, but it has other problems, like being slightly too heavy.