A walk on the sea

I got to the beach early in the morning and couldn't believe my eyes as i see people "walking on the sea", and no I do not mean walking at the beach. I could see a fort at a kilometre distance in the sea ,i summoned my superpowers and decided to walk on the sea myself.

               During the low tide one can walk or hire a chariot to the fort 
                whereas at other times, the only way to get there is a boat!!