I learned this technique from my SIL who was a caterer for 20 years.
I place a drinking glass into the sink; open tuna can and place upside with opened lid down onto the glass, and press down on the can until liquid drains out. I used to use the open can, squeeze until fingers hurt method, too, but there always seemed to be a little leftover liquid. The glass/sink method removes all the liquid. Whether you use canned tuna in oil or water, this method works great.