When you first take up triathlon, you’ll need to retrain yourself to breathe in the water. Go into the shallow end of a pool and, with your hands on the edge of the pool, stick your face in the water and forcefully breathe out. Once you can do this and still feel calm, go into the deep end and tread water. Then, perform a “sink down” by stopping treading and breathing out air forcefully; you’ll sink down in the water. The key is to do this over and over, calmly, to develop comfort breathing in the waterGo back into the shallow end of the pool and place your hands on the edge of the pool. Lift your feet off the floor and kick only as hard as you need to in order to keep your bottom and heels at the surface. Using a snorkel for this drill will help you stay calm, but like anything, using a snorkel will take about 10 sessions to get used to, so be patient. Once you’ve got this drill perfected, progress to the next step by taking one hand off the edge of the pool and kicking while turned 45 degrees on to your side. Make sure you’re still keeping your bottom and heels at the surface.Once you have the above drills nailed down, while still in the shallow end, push away from the wall and try to kick across the pool; first do this with your face straight down into the water, then on the way back, kick back to your starting spot while turned 45 degrees on your side. When you first start to do this drill, instead of trying to take breaths during the drill, stand up and reset to gather a calm mind, then restart. Eventually you’ll be able to kick across the pool with your face in the water, and your butt and heels at the surface of the water while turning 45 degrees on your side to catch small breaths; you’ll be calm as you float across the surface of the water without sinking legs.
https://www.active.com/triathlon/articles/how-to-stop-your-legs-from-sinking-in-the-swim