[Benjo]

You could look into using the IsPlayerInRangeOfPoint function for the player's position check. Perhaps find the centre coordinate of the ferry and then specify a range around that?
https://sampwiki.blast.hk/wiki/IsPlayerInRangeOfPoint

If you are willing to collect a few coordinates, you could use this IsPlayerInArea function:

pawn Код:

IsPlayerInArea(playerid,Float:max_x,Float:min_x,Float:max_y,Float:min_y)
{
    new Float:X, Float:Y, Float:Z;
    GetPlayerPos(playerid, X, Y, Z);
    if(X <= max_x && X >= min_x && Y <= max_y && Y >= min_y) {
        return 1;
    }
    return 0;
}

That could be used to make sure players are on the ferry, and are not on the dock. Just a few suggestions to get the ball rolling!