If a person on boat moves forward (from left to right of a boat)and if we draw the free body diagram we would observe that 1)mg is balanced by normal reaction 2)friction acts to the left side, on the man ,right? It's because friction always opposes relative motion 3)On the boat ,then,friction acts towards the right,but the boat moves backwards i.e to the left How should I understand this?
When you walk you are essentially using the ground to propel yourself using the friction force as a medium. However, since the Earth is so big it does not move. Boats, however, are much smaller, so the force caused by you walking is much more evident.
While these types of questions you are talking about can be solved by analyzing the forces, it would be easier to use the center of mass to analyze it (no external force = the center of mass stay at the same spot).
Hope this helps!