
Above shows where you need to be. Click the script button to pull up this window. If you don't know what button that is, it's the one that looks like a piece of paper with a green pencil on it.
Once here, click on "Game_Vehicle", as highlighted in blue in the image on the left side of the script screen. The section below line 54, in the red bracket, is where we need to work.
The game sets the boat as number 0, the ship as number 1, and the airship as number 2.
when 0;
@priority_type = 1
@move_speed = 4
when 1;
@priority_type = 1
@move_speed = 5
when 2;
@priority_type = @driving ? 2 : 0
@move_speed = 6
This is what you need to edit. when 0; means "when boat." When it's a boat, from the move_speed, you can see it's speed is 4, which is the speed at which your person normally walks. This can only be an integer between 1 and 6.
1 = 1/8 your normal speed
2 = 1/4 your normal speed
3 = 1/2 your normal speed
4 = normal speed
5 = 2X speed
6 = 4X speed
So from here, you just edit the number next to move_speed under the vehicle you want to change the speed of. So for example:
when 0;
@priority_type = 1
@move_speed = 3
when 1;
@priority_type = 1
@move_speed = 6
when 2;
@priority_type = @driving ? 2 : 0
@move_speed = 5
For some odd reason, I've decided to make my ship faster than my airship and the boat slower than my character walks. The code above says that the boat moves at 1/2 speed, the ship at 4X speed, and the airship at 2X speed.
I hope this helps anyone that wanted to change the vehicle speed and that this tutorial at least proves useful to someone.