Very few people are in reality negatively buoyant although some are close enough to it to be a problem.
When I was a young college student who was slim and athletic with less than 7% body fat I was one of those few who became neutral 3’ bellow the surface. Even at that state treading water isn’t particularly difficult if you stay calm and realize that it take very little effort to keep your head out of the water. You only need to float your head to breath! If you work any harder than needed to float your head you are wasting energy and may over exert yourself. Stay calm and minimize your effort. This will allow you to maintain a pace for a much greater duration.
A few year after college I had put on a few pounds, although I was still in great shape. I finally learned to do a dead mans float on my back. This is the most effective because it forces you to expand you chest cavity and give you lungs some extra capacity. On your back with your arms on the surface straight out to the sides, arch you back so your legs are 15 degrees or more pointed downward. Fill your chest and then breath shallow never releasing more than half your lung capacity. It may help to close your eyes the objective is only to keep your mouth and nose on the surface.
Here we are years later I’m not in great shape anymore but took my DMC 3 years ago and was able to float like described above with a 5 lbs weight on my chest (with some minimal kicking)