Odds of Getting In
Here's a question with a multi-part answer. First, we'll talk about the employment resources that come along with your instructor certification. You may not know that most dive instructor training facilities provide job placement assistance to their program graduates. School staff might even assist with your résumé preparation and job interview skills. You might also get access to free employment postings as part of the deal.
Beyond that, don't underestimate the value of networking. Get yourself to a local or regional dive show. Dress appropriately, take along a supply of professional-looking résumé (hint: get them proofread first), and spend some time schmoozing with prospective employers. No guarantees, but it makes sense to put yourself in front of the people who've got the jobs.
That covers your preparation. Now, what kind of SCUBA-related job can you get? That depends partially on your geographic location. In a smaller Midwest city, for example, you might be limited to working at the local dive shop and perhaps teaching a class at the local community college. The local economy might figure into the picture, too. Diving is an expensive sport when you consider the required SCUBA gear, air fills, and frequent travel to dive destinations. If you're setting up shop in a region that typically doesn't have a lot of discretionary income, you're going to have a tough road ahead.
Okay, let's assume you're in south Florida, and the picture changes considerably. You'll have lots of dive shops that could use your instructional services, and might need your retail sales and equipment service expertise as well. On the other hand, you might also have more competition for those jobs. You can set yourself apart by becoming certified to teach specialty diving courses. These might include wreck diving, underwater photography, or enriched air diving.
Now let's assume you've gone the resort route. You've hitched a ride down to the Caribbean to become a SCUBA instructor. What makes you more valuable than the next guy off the boat? Well, consider that resorts (especially dive resorts), are usually located well off the beaten path, which is directly proportional to the frequency at which equipment breaks down. If you show up ready to fix a cantankerous compressor or a diesel generator, you're golden. If you can also handle construction work, you're platinum.
If you've got lots of boating experience, you might find an instructor/deckhand job on a dive boat. If you've got a Coast Guard captain's license, you might be able to captain the dive boat. Before you pin your future on this option, however, you should confirm the resort's requirements for dive boat captain work.