How many people would fit hand in hand around the world and how many of those would have to tread water?

Im not sure of the exact circumference of the earth. The time it took me on a flight to Thailand from the UK was 12 hours and as commercial airliners can fly at around 500- 600 mph I estimate Thailand should be about 6-7 k miles away.

However from memory that wasn't exactly half way around the earth from the UK so would estimate that you would need another few thousand miles to reach this point. Assuming then that this is halfway mark is about 9k miles away. This would mean that circumference of the earth is about 18k. miles

When I stand with my hands in the relaxed 'hand in hand' pose (not my own hands in order that other people can hold my hands) I estimate that the distance from one hand to the other is about 4ft. As there are 5280 ft in a mile this would mean that a link of around 1320 people would fit into a mile distance.

If we multiply the 1320 (people per mile) x 18K (estimated earth circumference) the estimated people to circle the each hand in hand is around 24m (rounded up)

I think about 2/3 of the earth surface area is covered in water. This would mean that would mean that on average 16m from the 24m would be treading water.