Q: "A group of people are on a boat in a marina." Does this mean that "A group of people are in an airplane."?
A: no

Q: "Skiiers at the top of a snow covered hill." Does this mean that "Skiiers are preparing to ski down a snow covered hill."?
A: it is not possible to tell

Q: "Three people are on a sidewalk." Does this mean that "Three friends are riding the bus."?
A: no

Q: "A boy at a skate park prepares to slide down a ramp." Does this mean that "A boy is at a skate park."?
A:
yes