I dont know if this will help you:

Code (Skript):

1.  
2. on vehicle enter:
3.     event-entity is a boat
4.     set {vehicle.%player%} to "boat"
5. on vehicle exit:
6.     event-entity is a boat
7.     delete {vehicle.%player%}
8.  
9. every a minute:
10.     loop all players:
11.         if {vehicle.%loop-player%} is "boat":
12.             send "You're on a boat" to loop-player
13.  

 

Why using variables while skript has an expression for the vehicle which the player is riding ?

This is a condition to check if an entity is
riding a vehicle:

Code (Text):

1. %entity% is rding

To get the vehicle, use this expression:

Code (Text):

1. player's vehicle
2. vehicle of player

Here is an example in code

Code (Text):

1. on vehicle enter:
2.     set {_vehicle} to player's vehicle
3.     send "You are ridding %{_vehicle}%"