There are many different synonyms that can be used to describe the phrase "under sail." Some of the most common options include "afloat," "at sea," "on the water," and "in motion." These terms all imply movement and action, highlighting the way that a ship or boat is actively traveling across the water. Other synonyms that could be used to describe "under sail" might include phrases like "in transit," "on a voyage," or "journeying by water." In general, these types of synonyms focus on the idea of travel and explore the ways in which sailing can be seen as a method of transportation and exploration.