@GrungeQueef If it wasn't for GNU units converting to sane units for us, the braindead units would make it too hard to calculate.

45 mph == 72.42048 kph == 20.56384 metres / second
15 lbs == 6.8038856 kg

With assumptions, the calculation is; (15 lbs)(20.56384 m^2 / s^2) / (5 cm) (due to the water, we take a shortcut and ignore the immediate impact force and assume that the impact force is spread over 5 cm of movement due to the water).

That works out to be 2798.2803 Newtons of force, or 285.34518 kgf, which I reckon could give you a concussion if you get hit in the head, or a large bruise if you get hit in the torso - but most likely you won't die if you don't end up drowning.

There's a high chance I got the calculation wrong, so anyone is free to interject with the correct one.