Which buoy type would alert boats to an isolated underwater danger?

The isolated danger mark is specifically designed to indicate the presence of underwater hazards that may not be visible from the surface. Recognizable by its unique color and shape, it typically features black and red vertical stripes and may also have two spherical shapes at the top. This buoy communicates to mariners that there is a danger to navigation in the vicinity, signaling them to steer clear while navigating in its vicinity.

In contrast, a safe water mark indicates that there is safe water all around it, lighted buoys provide warnings or information through lights, and seasonal buoys usually mark areas that are only relevant during specific times of the year, which does not pertain directly to isolated underwater dangers. Hence, the isolated danger mark is the most appropriate choice for alerting boats to such hazards.