What action is required by vessels to prevent collision when they are in sight of each other?

When vessels are in sight of one another, they have a responsibility to prevent collisions through various actions, which include the appropriate use of sound signals. Sound signals, such as whistle blasts, are a crucial method of communication that indicate a vessel's intentions to others, especially when navigating in restricted visibility or when another vessel is nearby. By using sound signals correctly, vessels can inform each other of their actions, such as whether they intend to alter course or pass, which is vital in avoiding collisions.

The other options do not fully capture the essence of the International Regulations for Preventing Collisions at Sea (COLREGS) regarding vessels in sight of each other. For example, while altering course and speed might be appropriate actions depending on the situation, the decision to do so must be communicated through sound signals. Simply speeding up or maintaining the same direction without communicating with sound signals could lead to misunderstandings and potential collisions, which is why the proper use of sound signals is essential for safe navigation when vessels are in visual range.