Hypoxic training as well as drownproofing will help you out on that. With Hypoxic training all you do I swim normally but every lap try an go more and more strokes without breathing. Example: first lap breath every 2 strokes, 2nd lap breath every 3, 3rd lap every fourth and so on. This is traditional Hypoxic training for a swimmer. With the drownproofing, you don't actually drown yourself and there are several techniques. The one I'm thinking for you is very basic but effective. Go to the deep end of the pool, at least 10ft deep and no more than 15ft deep. You hold your hands behind your back and straighten your boy out and exhale to sink all the way down to the bottom. When you touch the bottom push up with your feet/toes to come up to the surface and take a breath, exhale and go down again. Do this for 5minutes to begin. When you get more comfortable work up to 10 minutes. With water treading use different techniques which you can google. Holding a gallon jug over your head while you tread water will strengthen your trunk and legs. I've done and still do all of these. This is 14 years of competitive swimming and being a swimmer in the military talking. And I'm just giving my personal advice. Hope it helps.