When going on vacation to a hotel, Resort or cruise ship be sure to bring ones, fives and tens.  You can never have enough ones.  

How much to tip and when it is appropriate is very location specific.  In general, cruise ships don’t require tipping.  They automatically charge you a daily tip and are billed at the end of the trip.  Any additional tipping would be all about getting the ship’s crew to come a running whenever they see you.

Bartenders are always good to tip.  They will remember you and get you your drinks faster.

How much to tip is location specific and changes over time.  In poor countries a dollar is usually good for most situations.  Somebody just made you and your companion a drink.  You were just handed a bathing towel so you don’t have to hike back to the resort’s special desk to get a fresh one.  Leave maybe two dollars left on the table at the breakfast buffet.

In Europe where tipping is not the norm they know you are an American and love to tip.  Often they’ll expect it from you.  Where the custom is 15% - 20% in the States, Europeans usually put a service charge on the bill making an extra 10% tip sufficient.

In any tourist location, it is customary to tip the Excursion guide at the end.  Not that every does it.  When I say excursion, I am referring to several hour long adventures like scuba, sailing and dune buggies.  If the activity is less than an hour and they don’t provide transportation, pick you up then return you to the resort, tipping isn’t expected.

Plenty of people don’t tip.  If you do, $20 for an afternoon’s exciting excursion is sufficient.  Ion a poor country if there are multiple guides it is better to hand each of them $5 rather than a $20 bill to split.  They may have trouble making change.