Swimming for triathlete

Getting ready for another triathlete season. Just finished the Houston marathon in PR time of 2:55. I need help with my swimming form technique. I really am a bad swimmer. Just learned about2 years ago. My buddy/training partner used to be a swim coach, but he seems to be greedy with his tips other than a tip here and there. Everyone that sees me swim says that my form is good. My problem is that I am just so out of breath after a 50 yd swim. If I try to do anymore than that I have to stop at the end of the pool to catch my breath for a few seconds and continue. My endurance is not a problem - I can run marathons! I want to know what is the most basic technique that is the most important in swimming to make me go longer without stopping and being out of breath. I am taking about 20 strokes per 25 yds and have been working really hard on breathing to both sides. Seems like I tend to breath only to the right. :x