There is no change in ocean water temperature when it is from the ocean surface to a depth of 1 km.
An educated guess is a “hypothesis”
Applying conservation of momentum
Quarterback mass = 80 kg
ball mass = 0.43 kg
Initially both together but horizontal velocity of both 0
initial momentum = 0
Final momentum = 15*0.43 - 80v
initial = final (law of conservation of momentum)
6.45 = 80v
v = 0.08 m/s