I loved the knight hopping into town for a better sword and shield while you were struggling out of the water to get on shore. I wonder what made you know that a secret place might be in the back of the fireplace. That was a good hunch you boys had. Your imagination has created a lively fantasy here. It was quite creative.
Next time try to describe the characters a little more.
Give readers a better idea of what the characters look like aside from just their age and eye colour. Your story could have been even more enjoyable if at the ending the two boys got into an argument in deciding on who should make the one wish and what it should be. The wish you chose to end your story with was pretty smart, though.