Beginner here, so please don't judge me too harshly, but here's my pitch:
[ぼく は] ことし の なつ やすみ に うみ へ およぎ に いきました。
[Boku wa] kotoshi no natsu yasumi (ni?) umi e oyogi ni ikimashita.
Assuming I got the general idea down, I'm not sure whether I need a "ni" after the declaration of time here.
