(1) go to sea意为“去航海，当水手”。

• Many British boys want to go to sea when they are old enough.
  许多英国男孩长大后都想当水手。
• He announced that he was going to sea for another voyage.
  他宣布要在海上再做一次航行。

 (2) go out to sea意为“出海”；put (out) to sea也是“航海，出海”，两者意思基本相同。

• The ship was unable to put (out) to sea/to go out to sea on account of the gale.
  由于飓风，这艘船未能出海。

 (3) go to the sea/seaside意为“到海边”。

• We're going to the sea/seaside to spend the summer vacation this summer.
  我们打算今夏去海边度假。