Whenever a shot is fired by any gun, the particle effect on the barrel and the resulting particles from bullets landing results in a conflict with water. This is even more apparent when using shaders with Optifine, when it just results in a black box with the image as a particle.

Without Optifine and shaders everything works fine. Optifine is often not compatible with mods.

Try to using the newest Forge and Optifine, I'm using the latest version of Optifine and found no visual problems...