Diesel/Engine Training?

Sounds like you're talking about the small boat arena (under 80'). For the most part I've found they look for no engine qualifications or certifications. They don't seem to even enquire past 'Do you have a license'. However to build a reputation and keep working, the more you know the better. The more you know, the more indispensible you become. Borg Warner for one has a decent diesel course from what I understand. Once you get over about 80' you'll be entering engineer territory, and certifications will be very important. The engineers here can jump in and advise if you're interested in becoming an engineer.
My policy (in the small boat arena) has always been to be able to fix most things in an emergency and to be able to oversee work being done, but to leave all but the simpler tasks to those who work every day as (and charge as) mechanics. Try to do everything and you'll do nothing well, and attract penny pinching clients who want their mechanical work done without paying a mechanic's rate.

Keep getting that wheel time and sea time. Mouth closed, ears and eyes open. Remember that the right way to do things is the way the captain likes them done, and work to become the "go to" guy. The engineers here would be the best to advise on certifications and education, especially since you're on the left coast. I went to the School of Hard Knocks. Good luck.

I do a bit more than NYCAP as a Captain and fix whatever I can on a trip to keep going without having to have a delay waiting on a mechanic. I'm very familiar with electrical, plumbing, engine stuff, etc. Now, I also know where to draw the line. I don't go inside the valve covers on a motor and I know where to call a professional. I learned most of what I know on the job, but had worked for my dad in high school/college who owned an auto mechanic shop and body shop. A lot of that transferred over. Watching professionals when they came out also helped with a lot of the other stuff. Most Captains are not expected to know more than what NYCAP fixes. However, it comes in handy, especially if you're in some small island in the Bahamas. It might be best to take a course offered by a major engine manufacturer and whichever one is most dominant in your area like CAT, MTU etc.

What Capt. J described is the school of Hard Knocks. But it's a very long course. I've been attending since I was 14. Since I could never afford to stop working to take courses and pay for diesel & gas, 2 stroke and 4 stroke, and general mechanical courses I always hoped to land an employer who would invest in me. But of course such employers are rare. If you can afford to do that I recommend a general diesel course as opposed to a specific manufacturer's course unless you intend to become a mechanic. Murphy's law dictates that if you study say CATS the boat you work will have something else. Also, unless you take continuing courses, a lot of what you learn will be obsolete in short order.

Quintin, I thnk this is an excellent way to set you apart. As an owner I'd be more concerned in exp and knowledge than a certification. I think Capt J's suggestion of looking at local Cat training classes would be great. At the end of the day you have to pick a platform ( Cat, MTU, etc) that you want to be deeply familiar with. I think Quinn Power is the CAT guys for your area.

I will say that most of their classes are truck engine focused, but maybe they can point you in the right direction.

Replacing water pumps, exhaust and heat exchange components should all be in the captains wheel house.

This sound fun.. http://www.pantropic.com/marine_class.php

As an owner I would be interested in the combination of experience and training. As someone new to the field, anything you can do to increase either is to your benefit. You can't magically gain the experience that Nycap or Capt J has overnight. But you can get some training to accelerate the process.

Without some some knowledge or experience I would not consider you as a captain, but would perhaps as a mate or deck hand if I had such positions.

In Fort Lauderdale, our community college offers a great program, http://www.broward.edu/academics/programs/marine/Pages/default.aspx

I suspect there are similar programs in your area.

Our job descriptions do include a requirement of basic maintenance and engineering knowledge.