I first swam when I arrived there, and then I swam the next morning.

This is a little picky, but a comma should be in there!

Theat feeling that when you are at athe beach, see looking at the end of the sea, blowing against theless sea, feeling the blowing sea wind, and swimming in the sea is very nice.

So I suddenly had anthe idea to find a medium-sized coastal city where I could see the sea, blow againstfeel the sea wind, and swim in the sea all year round.

But theHowever, houses in coastal cities are all expensive.

After I buyIf I bought a house in a coastal city, I would spend my time on athe beach, swimming, blowing against the sea wind and seeing the end of and feeling the sea wind and looking at the sea.

I first swam when I first arrived there and then I swam the next morning.

"first" goes with "arrived" so is near it in sentence. You don't need the second "I swam" as you have it earlier in the sentence.

The feeling that when you are at a beach, seeing the ewind of the sea, blowing against the sea wind's edge and swimming in the sea is very nice.

Where water meets land it is an "edge" not an "end" e.g. sea's edge, lake edge etc..
It's possible I am misinterpreting what you are trying to say here "seeing the end of the sea, blowing against the sea wind"?

So I suddenly had anthe idea to find a medium-sized coastal city where I could see the sea, blow againstthe wind blowing on the sea wind's edge and swim in the sea all year round.

medium-sized rather than medium to describe how big a city is in this instance.

This kind of place can only be found in the southern part of China, like Guangdong, Guangxi and Hainan provinces, because coastal cities in the north of China are very cold during the winter.

"southern" is the adjective.

I hope I can find a small one that is not thatoo expensive.

"not too expensive" for a house that you can afford to buy / rent.

After I buy a house in a coastal city, I would spend my time on a beach, swimming, blowing againstwatching the sea wind and seeblow and watching the end of the seawater's edge.

The feeling thatof when you are at a beach, seeing the end of the sea, blowing against the sea wind and swimming in the sea is very nice.

So I suddenly had an idea to find a medium coastal city where I could see the sea, blow against the sea wind and swim in the sea all year round.

I'm not sure what you mean by "blow against the sea wind" but I've never heard that statement. I'm not sure how to correct it because I'm not quite sure what you mean.

But tThe houses in coastal cities are all expensive.

When speaking out loud, it's okay to start a sentence with "but" but in written language, it's "improper" to start with "But".

After I buy a house in a coastal city, I would spend my time on a beach, swimming, blowing against the sea wind and seeing the end of the sea.

Again, I'm not sure what you mean by "blowing against the sea wind"

The most exciting thing [during my trip / during this timerip] was swimming in the sea.

Note: there’s nothing wrong with what you wrote, grammatically, but we wouldn’t say “during my trip this time.” For 我这次的旅游 we’d say “during my trip,” “during this trip” or “the most exciting part of this trip was….”

The feeling that when you are at a beach, seeing the endxpanse of the sea, blowing against the sea windfeeling the sea breeze in my face and body and swimming in the sea is very nice.

I’ve edited my corrections based on your comments.

So I suddenly had an idea to find a medium-sized coastal city where I could see the sea, blow againstfeel the sea wind/breeze and swim in the sea all year round.

After I buy a house in a coastal city, I wouldill spend my time on a beach, swimming, bloweing againstcaressed by the sea windbreeze and [seeing the end of the sea | gazing at the horizon].

It would be the ideal life for (me in) my old age.