Misc: In 1962 Mariner I launched to Mars falls into ---------- Ocean. ?

Questions

What is the only english word that ends in the letters 'mt'?

What is the only n.y.c borough that is not on an island?

What is the only planet that is less dense than water?

What is the only river that flows both north and south of the equator, crossing it twice?

What is the only sport that has a rule against left handed players?