What is indicated by a white marker with black vertical stripes?

If you see a white marker with red vertical stripes, it means the water is safe.
The Safe Water marker

These are white with red vertical stripes and indicate unobstructed water on all sides. They mark mid-channels or fairways and may be passed on either side.

Commonly known as a "nun" buoy
Safe water markers are white with red verticals stripes. They usually run through the deep middle of channels or rivers and indicate safe boating water. They may be passed on either side.