Your code

@EventHandler
public void onBoatCollideWithEntity(VehicleEntityCollisionEvent event) {

   // You get the entity that was hit.
   Entity entity = event.getEntity();

   // You get the vehicle that hit the entity.
   Vehicle vehicle = event.getVehicle();

   // You ignore this event if the vehicle is not a boat.
   if(!(vehicle instanceof Boat)){
      return;
   }

   // You get the primary passenger of the vehicle.
   Entity passenger = vehicle.getPassenger();

   // You ignore this event if the boat was driven by anything other than a player.
   if(!(entity instanceof Player)){
      return;
   }

   // You get the player instance.
   Player player = (Player) passenger;

   // You check what entity was hit by the boat.
   switch(entity.getType()){
      case BOAT:
         player.sendMessage("You hit a rival!");
         return;
      case ARMOR_STAND:
         player.sendMessage("You hit an armorstand!");
         player.sendMessage("You have received a boost!");
         // add boost here.
         return;
      default:
         return;
   }
}