Dont ask. The answer is a resounding "Yes"! You should tip. When you go on a chartered and chaperoned dive through a commercial diving shop, it is customary to tip both the Captain and the DiveMaster. Gratuities are voluntary, incumbent on your generosity. Even so, it is appropriate to offer a tip for your scuba crew.
If you are going out for one day, it is important to bring some cash on the boat for tipping. If you are diving with the same DiveMaster and Captain for an entire week, it is acceptable to offer a tip at the end of the week.
If you are arranging a variety of dives through a dive shop, you may not have the same Captain or DiveMaster on each dive, so daily tipping is a better idea unless you are certain that your crew will be consistent for the entire duration of your stay.
One perspective is to compare the roles of Captain and DiveMaster to other industries where tipping is expected. Your Captain is analogous to a taxi driver, transporting you safely to and from your scuba destination.
An acceptable tip for a taxi driver is 10% to 15% of the fare; a similar rate for the Captain is appropriate. On a single-tank dive for $65, your tip to the Captain should be between $6 and $9 USD.
Your DiveMaster should be more generously tipped with 15% - or more, if the service was exceptional. On a $65 single-tank dive, tip your DiveMaster between $8 and $12 USD.
Do not skimp your tip due to variations in the local currency. If you are traveling in an area with an embarrassingly favorable exchange rate, do not lower your tip just because the local economy is cheap. To a boat Captain in Florida, $15USD is an acceptable token of gratitude for a nice trip.
To a Captain in rural Honduras, $15USD is a generous gift. Give your generous gifts where they are most needed to DiveMasters offering good service in economically challenged regions. Keep in mind that some crews derive much, and sometimes nearly all, of their income from gratuities. Stay cool, and keep on diving!