It is correct, the action is paddling, where you move the water backwards, and the reaction is the boat moving forwards.
A droplet of pure mercury has a density of 13.6 g/cm3. What is the density of a sample of pure mercury that is 10 times as large as the droplet?
Answer: In this case the density will remain constant for both droplets. The reason being that volume will not change the density of the material. The only way of changing it is by changing its state. If you increase the volume then the mass will also increase. Leaving the density the same.
I hope it helps, Regards.

1 ys is the smallest unit of time.
Answer:
The answer to your question is F = 16 N
Explanation:
Data
mass = 8 g
vo = 800 m/s
distance = 20 cm
force = ?
vf = 0 m/s
Process
1.- Calculate the acceleration of the bullet
Formula
a = (vf² - vo²)/2d
Substitution
a = (0 - 800)/2(0.2)
a = -800/0.4
a = 2000 m/s²
2.- Convert 8g to kg
1000 g --------------- 1 kg
8 g -------------- x
x = (8 x 1)/1000
x = 0.008 kg
3.- Calculate the force
Formula
F = ma
Substitution
F = (0.008)(2000)
Simplification and result
F = 16 N