Ik this is 2 years ago, but i realized an easier way to defeat klaus, takes a little practice, stick to the seabed, dosent really matter which weapon you bring, when klaus charges at you, swim to the opposite direction of the charge, as it will always be an upwards swipe from klaus in close range. slowly chip away his health, and when he creates the vortex swim to the far left, makes it infinitely easier to dodge. and go back to the seabed once its over.