I'd like to talk about this topic because I have experience with it.

I was not prepared forlanning to swimming in advance, so I didn't have a swimsuit.

Swimming isn't really something you prepare for unless you were to take lessons or something, so 'planning to swim' or 'expecting to swim' is better here :)

Maybe tens of minutes later, I went back to the beach.

"Tens of..." is not an expression in English, which is confusing because "hundreds/thousands/millions of..." is commonly used. If you mean somewhere between ten minutes and an hour, you would just say that rather than "tens of minutes".

I just gowent around the beach and basked in the sun until my body driesd.

I used to buyhave a swimming pool membership in a five-star hotel, but I didn't swim too much there because I was lazy.

The difference between the two kinds of swimming is the feeling.

When you swim in a pool, you don't fear it because you know that it's very safe: the water in the pool is not deep, the pool is not so big, and salife guards are watching you, so you won't encounter any danger.

This is a very well-written sentence by the way, it sounds like something a native speaker would say :)

The sea is endless, the waves are strong, and the water is so deep, so you have to be very careful to watch out.

Even if you know it is safe, you still have an fear of the unknown fear.

I might have misunderstood you here -- it would also make sense to say an "inexplicable fear" if that is more what you mean. However, "unknown fear" doesn't sound natural.

I like to talk about this topic because I have experience.

not sure what you mean by "I have experience". maybe "It was an experience" or "I have experience in this"

The first time was in Sanya in 2010, and the second time was in Weihai in 2017.

When I was at the beach of Yalong bBay in Sanya city, many people swamwere swimming in the sea.

Maybe tens of to twenty minutes later, I went back to the beach.

tens of minutes -- not used in english

I just gowent around the beach and basked in the sun until my body driesd.

I used to buyhave a swimming pool membership in a five-star hotel, but I didn't swim too much there because I was lazy.

or "I once bought a swimming pool membership"

"used to buy" would mean that you used to regularly buy something

The difference between the two kinds of swimming is the feeling.

When you swim in a pool, you don't fear it because you know that it's very safe: the water in the pool is not deep, the pool is not so big, and salife guards are watching you, so you know that you won't encounter any danger.

The sea is endless, the waves are strong, and the water is so deep, so you have to be very careful to/ watch out. / be alert / be cautious

Even if you know it is safe, you still have an fear of the unknown fear.