Cash tip, directly to captain as it's for the crew not the charter company. Same as when you tip for a meal. 15% probably average tip...20% for great crew. If it goes like it generally does, you will regard the captain and cook as friends pretty quickly. So...treat them as such. The horror stories come from the folks who charter treating crew like "hired servants".

As for destinations Twanger is right, your schedule needs to be somewhat flexible due to winds and weather. If it works sail to Anegada and spend a day or two there. That's my best tip.