Think of your Boat Captain and Dive Master. They work hard, and taking a line from one of our whitewater rafting buddies, Stu, and paraphrasing it a bit; These folks spend a lot of time with you, and perhaps safe guard your well being, so if you tip a couple of bucks to a waitress for spending a few minutes with you, think about these professionals working to make this safe and enjoyable. Typically $10+/day Dive Master & $5+/Day Boat Captain per person.  If you fail to come back on the boat, don't worry about this.

You have staff about the grounds. A little appreciation can go a long way.

The airport departure tax is not included. This includes Curacao and Bonaire.  Have currency with you, it is typically $25 US. No credit cards for this. Without paying, your extended stay and additional return costs are not covered in the price of this trip.  In fact, I'm not sure how you get back or what room they place you in.

There could be additional dive expenditures.  If you decide you need additional equipment, it can be rented. If you equipment breaks, you might get it fixed for a fee or need to rent.  Enriched Nitrox (if you don't know what this is, you won't be using it) is available for additional costs. If you are a scuba diver, you know about this stuff. 

Additional dives outside of what is provided may cost extra.  Diving the town piers is a fee and cost of a dive master (their rules). Security clearance and advance planning is required.

There is sight seeing about the island.  Bring money to cover these excursions.  Don't wonder around alone.  Stay in a group.