In this situation, sooner or later you’ll have to move in the water. The style of swimming you choose will determine how long your strength will last. Remember this rule:
In calm water, it’s better to swim on your back because you’ll get the opportunity to relax as much as possible and breathe evenly.
If waves splash around you, it’s better to swim on your belly. For example, try the breaststroke. Take a breath of air, then lower your head into the water and exhale. If you don’t have to keep your head above the water’s surface all the time, you’ll save more energy.
Find something that floats