Analysis: Taking man and ship as a system, we can know from the meaning of the question that the resultant force on the system is zero, so the momentum of the system is conserved.
Suppose that the walking speed of people is V 1 and the speed of ships is V2, then there is m*V 1-M*V2=0.
The speed in the above formula holds for instantaneous speed and for average speed.
If both sides in the above formula are multiplied by time at the same time, you can get
m*S 1-M*S2=0
S 1 is the distance that people move relative to the ground, and S2 is the distance that ships move relative to the ground.
Because people and ships move in opposite directions relative to the ground, there is s1+S2 = L.
Get M *(S2)= M * S2.
Then the distance the ship moves relative to the still water surface (the same as the effect relative to the ground) is S2=m*L/(m+M).
That is, S2 = 80 * 4/(80+600) = 8/17 = 0.47m. ..