In a word, it's hard. Currents will play havoc with swimming a straight line. Even if you are accurately holding a compass heading, the drift will traverse you sideways.
As noted above, distance measurement is fiendishly difficult.
Here's an old tool I used "back in the day" to try and figure out where I was going and the most direct route back from a wreck dive:
http://members.aol.com/cbjpegs/scubanav.jpg
It worked like a pilot's E6B computer, rotating the dial to align azimuths under a fixed grid.
In general, I considered it good work if I could just hold an azimuth and hit the beach within 20 degrees of my intended arrival point. we didn't have modern "consoles" so we used wrist mounted compasses. Mine was on my left wrist. When trying to hold an azimuth I would extend my right arm straight ahead, then grab my right elbow with my left hand. That put the compass in front of my face. I would try and keep the compass needle centered while swimming forward with flutter kicks. It helped to find landmarks on the ocean floor by sighting a line off the extended right arm.