Here is a quick treading drill you can do as a rest set in between fast pace swim sets.

I usually do a 3 minute tread with these three kicks (1 min each): NO HANDS

Start off with 1 min of scissor kick - alternating left / right leg kicks.

Then do 1 min breast stroke kick - vertical swimming 

Then try to start alternating your breast stroke kicks into left / right kicks which is a way to start your egg-beater motion.

Then start your swim set again - resting with tread with no hands.