Middle of a catastrophic event, just lost his wife, on the verge to die. It's kind of easy to lose your mind in those moments. At first, he was probably using them to stay afloat, then when they reacted and fight him, he fight them back too.
It was not a rational action.
The sensation of drowning or suffocating easily causes panic in most people. People who are hanged short-drop (ie. the neck does not break) will start kicking wildly before passing out. They know ahead of time that this activity is pointless, and they might indeed have made up their mind to fight the urge to do so in an effort to die with dignity, but they still end up doing it - because when you are unable to breathe, survival instinct kicks in and orders your body to try anything and everything to get out of the situation.
Phillip wasn't intentionally trying to drown Sondre or Idun, he was trying to climb to safety. When in water, panicking people often try to do just that: grab hold of anything in order to pull themselves above the surface. If grabbing hold of another person, climbing that person will result in pushing him down and below the surface. They're not thinking "I've got to kill him" or "it's him or me", they are simply thinking "AIR!"