If you enter the water as a diver and don't hold you SDAR (or other rifles/mps) in your hand, you are not able to switch to your weapon, as long as you are swimming or diving.
Additional Information and thanks to CXN2615:
If you hold the SDAR in your hands while swimming and "accidently" press the key for "switch gun / handgun" (; by default), you will still hold the SDAR in your hands but aren't able to shoot.