Means you are more compact than you think as well. The more muscle you have, and the denser your bone, the more you sink. Also, if you're afraid of water, you likely tense up and pull everything in. My recommendation is go to a pool and go to where you're only up to, or slightly below, your chest, no deeper, and learn, first, to tread water. Once you learn to tread water, then you learn to swim.