The best thing that happened (to me) in this year is my vacation. It's the first time I see the sea. It was wonderful. I have swum a lot (yes that is correct, it's the present perfect).

The best thing that happened in this year is my vacation. It was my first time when I have seen a sea. It was wonderful. I have swum a lot.

The best thing that happened in this year is my vacation. It's my first time when I have seen a sea. It was wonderful. I have swam a lot

The best thing to happen to me this year was my vacation. It was the first time I had ever seen the sea. It was wonderful. I swam a lot. You can also say "happening" in place of "to happen". You can say "a sea" but it is more common for us to say "the sea" and "the ocean". It would sound strange, for example, to say "I swam in an ocean this summer". Instead, you would say "I swam in the ocean" or "I swam in the Atlantic Ocean." You can either say "It was the first time I ever saw the sea" or "It is the first time I have ever seen the sea" or "It was the first time I had ever seen the sea"