"坐船到国外去" can be translated into English as "go abroad by ship" or "travel overseas by boat".
Both expressions convey the idea of traveling to a foreign country via water transportation. The choice between "ship" and "boat" may depend on the context or the size of the vessel being referred to, but in general, both are acceptable. "Go abroad" and "travel overseas" are interchangeable in this context, both meaning to leave one's own country for another.