When it comes to finding synonyms for the phrase "old sea dog," there are plenty of options to choose from. Some of the more commonly used ones include "sea veteran," "nautical sage," "salted sailor," and "weathered mariner." Other possibilities include "ocean wanderer," "sea wolf," "sea-faring elder," and "salty dog." Each of these phrases conveys the notion of someone who has spent a great deal of time at sea and has acquired a wealth of knowledge and experience as a result. Whether you're writing a story, composing a poem, or just looking for a colorful way to describe a seasoned sailor, one of these synonyms is sure to fit the bill.