Hello Elgar!
Wellcome. Well, you shoul stop the ship/boat like a 1-2 may 3 "meters(when it lookslike 2-3 meter), away from the person in the water. Then you in most missions are told to wait (about 10-20 seconds), and then is the persons saved. And the boat have do less than 1 knots speed, the best is of cource to stop completly. And in some mission where you sail a big ship, you have to deploy a smaller boat to pick him up, and in some you should use the ship you sail with, but that thing you are told in the mission. I hope you understand.