"坐船到国外去" can be translated into English as "go abroad by ship" or "take a ship to go overseas".
Both expressions convey the idea of traveling to a foreign country by boat. Here are some examples in context:
We plan to go abroad by ship next summer. （我们计划明年夏天坐船去国外。）
They decided to take a ship to go overseas for their vacation. （他们决定坐船去国外度假。）