Can you please improve my story?

I am 11 and apologize if you find my writing boring for your level, but if you could guide me and help me with the mistakes I will be very grateful.
Thanks!

My story:

There, on the soft damp sand stood the creation of my solitary time: a half-finished sand castle. If one were to carefully trace the water droplet trickling from the side walls, they would see it heading towards the millions of grains of sand, each particle reflecting the rays of the sun individually.
I was always fascinated by the new things I found at the beach, but the water droplet pointed to me a very mysterious object. I stared in astonishment as it vanished into the nearby sand.
It seemed as if a large gold coin was staring back at me. Furious, I kept digging with my shovel until I saw it vanish. I thought I might have made it go further in the sand, I kept digging. I dug until the sun started to move towards the horizon. I couldn’t give up! The coin was too precious.
There in the darkness of night, I stood staring at the huge hole I had made in the ground. I must leave now, I thought. Before I could turn around to go home, a rock made me lose balance….
***
My clothes were soaked in seawater, and the salt burnt my eyes. The sound of children playing was replaced by the violent waves splashing on the rocks. I had fallen into the pit! I had to get out of here before the waves brought sand to bury me alive! “Help!,” I yelled. Some water from the waves was trapped in the hole with me.
I kept sliding and slipping, with nothing to hold onto. This went on for over an hour and no one heard my cries for help. My hands were hurting a lot by now. The water was already up to my knees. Just as I was about to give up, I heard footsteps.
A sound from a distance told me someone was running in my direction! Was I dreaming about one of the Harry Potter books I had read? I could see his outline, he was tall and sturdy. Darkness covered him. What was he doing at the beach at midnight? What if he was a thief?
Frightened, I carefully sat down and lowered my head into the water. Being a good swimmer I could hold my breath for a very long time. I thought I would count till 100 and by then he must have left and it will be safe to come out of the water.
***
I was wrong. There stood the tall man, his hand trying to reach out for me. I closed my eyes shut as he lifted me out of the pit. “Th – thanks,” I stammered struggling for my voice, “Who a-a-re y-ou?”
Without speaking he gave me something that made my hands feel very cold, then he turned around and left. I stood there frozen in the chilly air watching the stranger jog back to the road. Too terrified to stay there any longer, I ran back home. Before I opened the front door, I looked at the cold thing lying in the center of my palm. The light of the front porch reflected off what was to be the most splendid gold coin in my beach items collection!

Letting us know that you're 11 does help, but you certainly don't need to apologize for anything.

Compliments on your spelling excellent and grammar. Adding a line of whitespace between paragraphs does help with readability, just FYI for future stories.

I only have two real problems with this story:

1) The pit itself. In order to dig a pit big enough to be trapped in, he'd have to be digging inside the pit; then he had to have climbed out in order to fall back in. So how was he trapped? Do you see the logical problem? It's also a bit of a stretch to have one child with a toy shovel dig a pit that big by himself in one afternoon and evening, plus he'd be both tired and hungry long before he was finished. Speaking of which...

2) Where are his parents? He's at the beach alone, gets trapped in his hole, doesn't get home until after midnight (how does he know the time?) and nobody ever looks for him? Also, he's a kid at the beach by himself digging a crater of dangerous size, with no parents about, and nobody says anything?

These issues don't have to kill your story, but you should give some thought to ways of fixing them.