In a head-on situation, what is the action that both vessels must take?

In a head-on situation, the correct action for both vessels is to alter course to starboard. This directive comes from the rules established under the COLREGs, specifically Rule 14, which applies when two power-driven vessels are approaching each other with the potential for a collision.

By both vessels altering course to starboard, they create a safe passage along the port side of each vessel, effectively reducing the risk of collision. This maneuver is essential for ensuring clear communication in terms of navigational intentions and enhances safety on the water.

The other options do not comply with the COLREGs requirements for head-on encounters. Maintaining course and speed could lead to a collision, as neither vessel is taking proactive measures to avoid one another. Increasing speed or changing speed differently could create confusion and uncertainty about each vessel's intentions. Sounding horns simultaneously does not address the immediate need for course alteration to prevent a collision, as it does not provide a means of navigation. Therefore, altering course to starboard is the only appropriate action both vessels should undertake to comply with the regulations and ensure safety at sea.