Each crew member has its 'cost' and your pirate ship has the maximum number of cost it can carry. If the total of you pirate's cost exceed ship's total cost, you can't take them on your journey.
1. Total of your pirates' cost.
2. Maximum cost capacity for your pirate ship.
3. Individual cost of each pirate.
This may sounds little complicated but simply put.
Cost are like weight. If you surpass your maximum weight, the ship can sink!
You might ask,
"Then what do I have to do to take better pirates from my inventory?"
The answer is simple. Just upgrade your pirate ship and the maximum cost capacity will increase.
Simple right?! Now go upgrade your ship and take stronger pirates with you on your journey!