I float so much better now that I am fatter. When I was a skinny kid I could not float easily, and there was my very overweight dad lying on the water like it was a bed. Now, I can do the same.
Staying afloat takes very little effort in calm and comfortable waters, we used to practice treading water in camp. In the ocean or fast moving waters, you might deal with waves, undertow, cold water, hot sun, swallowing salt water, and getting turned around so you swim deeper down into the water rather than up to the surface and wind up inhaling water.