If you're two people in the same boat, the passenger can only start fishing while the boat is in motion, the one in front can do it at any time (boat will move to accommodate his click).
To replicate:
- Have two dudes with fishing poles enter a boat
- Boat into position and stop
- Passenger tries to cast a line and fails
- Driver clicks with fishing cursor some distance away
- Passenger casts his line close to the boat while the boat is moving
- Both will keep on fishing once the boat stops