
By "default" here, you mean the speed it goes at without the plugin? I just tested again and can't seem to reproduce that behaviour. Some things to check:
- Are you testing from a new game? If not, the vehicle speeds may be being loaded from the save.
- Have you renamed the plugin file? If so, the parameter values will not be recognised.
- Are you using any other plugins? If so, could you list (or screenshot) the plugins you're using, in the order they appear in the Plugin Manager?
I imagine it's point 3, since I can't identify anything in my plugin that'd cause the "only works once" thing...
For testing, I set the boat to speed 1, the ship to speed 8, and the airship to speed 4 in the plugin parameters. I started a new game, set the player speed to 5 (move route), walked around at dashing speed, got in the boat, and went rowing excruciatingly slowly across a small lake. I got out and was back to move speed 5. Rowed back over, really slow; got out, back to speed 5. Similarly with the ship and airship, except their speeds were more tolerable. Maybe I'm missing something?