Few days ago, I was at the sea and I wanted to climb on a rock. Suddenly, I saw an orange thing holding on my foot . I looked closer and I realized  that it was an octopus. I was surprised right after it had disappeared in water. It was wonderful, I didn't knew that there were some octopuses in the sea near at the my city . This day was wonderful.

Hey, I changed some details to make your text sound a bit more neat. Remember that when you tell a story about something that happened in the past, it is good to use a lot of Past Simple and Past Continuous.