If you sit on the rear gunner or commander position in the Speedboat HMG or Speedboat Minigun you can experience a floating bug if you get out or eject out of these positions.
The charakter gets stuck in a floating position for a few seconds and slowly sinks into the water.
That issue only appears, if the Speedboat is standing still.
Duplicate of http://feedback.arma3.com/view.php?id=27190, but this one is much easier to understand and duplicate.
@DJPorterNZ: My ticket isn't a duplicate. The other ticket poorly describes a problem that occurs if you get/eject out of all three kinds of the motorboat with an unarmed diver. The diver got stuck on the exit point and you are only able to dive up and down but not forward or backwards. Try it for yourself to see the difference.
@OfotViking: According to the video, both tickets may be related. But the Speedboat problem does not depend on how close you are to the shore.