Yes, gratuity is normally at the discretion of the guest. As long as the guest feels he has received good service, a great meal and proper attention, they leave a gratuity/tip. At most establishments, these are divided equally between all the waiters etc in a restaurant. However, at All-Inclusive hotels, gratuity is not usually left but is always appreciated. Something as little as $10USD, or as much as $100USD is given daily. It depends how satisfied you are. In the event that you want to give something to a specific waiter, you need to do it discreetly. You may need to do this if this specific waiter was outstanding from the lot.

 On the otherhand, most restaurants have 10% service charge and a few have 15% service charge which is non-negotiable. This will be included in the total amount you are asked to pay.   However, a few also leave open the credit card total in the hope you might add a second service charge. It won't be turned away!