First, let's look at what you bring to the table. If you've racked up extra certifications, such as night diving, deep diving, or dry-suit diving (just a few examples), you're much more valuable to a potential employer. Translated: You can make that employer more money, not only from teaching the higher-level classes, but from sales of additional dive gear and accessories at the employer's dive shop. If you've mastered web design skills, you can help jazz up your employer's website so it stands out in the local dive market.
Now let's look at your potential workplaces and how each employer might compensate you. At a dive shop, you might get a set hourly wage or base salary, plus commissions on courses you teach and gear you sell. Depending on how prosperous the dive shop is, you might also get some bennies out of the deal. Examples might include gear discounts, good rates on dive travel packages, or some type of savings plan. If you're teaching at a dive resort, on a cruise ship or aboard a live-aboard dive vessel, you'll likely receive a lower salary. However, you'll probably get free room and board and maybe even some nice tips. All the more reason to be nice to your students.
Perhaps you've decided to float along as a freelance dive instructor. Yes, you can command a pretty nice rate for your services. However, you'll need to rent your classroom, pay for pool time (although you might negotiate a package rate), and pop for really good liability insurance. Yes, this is one profession where you don't want to skimp on the liability coverage. You'll also need to keep your class' dive equipment maintained and cover travel costs for students' certification dives. That further cuts into your profit margin.
With all that said, let's look at the numbers. Within the United States, a SCUBA instructor will probably make between $35k and $40k per year, which likely includes all his income sources. At dive resorts, newbies gross around $20k annually, while seasoned instructors make about $22k per year. On a cruise ship, expect to make about $1,700 per month; while your live-aboard dive instructor salary comes in around $2,000 per month. If you manage to land a private yacht gig, you can expect to rake in close to $3,000 per month. You should also remember the room, board, and tips you'll receive through your excellent customer service. Of course you'll report them, right? We thought so.