To the "On Activation" field of the
previous waypoint:
boat setFuel 0; leader group player assignAsDriver boat; player assignAsgunner boat
...and then you make the "GetIn" waypoint to the boat. To the "On Activation" of the "GetIn" waypoint:
boat setFuel 1
This way, the leader goes to the driver position and orders the player to get in as gunner. Leader will not go to the next waypoint until player is aboard.
There might come an annoying sentence from the leader: "Out of fuel" but the boat has to be glued to it's position until the player really is aboard. Why? Because the leader might foolishly play with the throttle and make the boat go a few meters away from the beach leaving the player wondering "What now?".
After the boat trip is over, maybe
"unAssignVehicle _x" forEach units group player
would be good to put into the "On Activation" field of the waypoint that made them come out of the boat.