Note: Your composition seems to happen this coming summer vacation. It's what you want to do soon. Thus, the verbs are in FUTURE TENSE.

I want my summer vacation to be great. On summer vacation, my family is going to the beach and i swim.  We will stay in a hotel near the beach.
I want to go swim in the pool. My friends and I will play in the water.
It's so cool. So, I want my summer vacation is great.

Note: When writing a diary, use the PAST TENSE of the verb. Below is a sample diary based on the composition you made.

My summer vacation was great. We went to the beach. We stayed in the hotel near the beach. I swam in the pool. My friends and I played in the water. It was so cool.

Eric, kindly read the two compositions above. Thank you for submitting your homework! :) See you on Tuesday next week!