Actually, I did some looking up of see-bound cargo ships of the era to see what they generally looked like, and I found that most are generally armed because of constant piracy in inhospitable waters. So, most water ships are far from unarmed. They still can't hope to put up a real fight against a well-armed, determined enemy. However, it is generally enough to scare away anyone who isn't prepared.
As for crew, I imagine that the sky fleets see their share of trained men quitting or getting canned in onesies and twosies. Not all pirate crews defect en masse. In fact, it's possible that some crime syndicates have access to trained personnel from all walks of life manning ships they managed to get ahold of one way or another.
If piracy is common, it's possible to see hiring halls for pirate ships looking to replace lost crew members. It's also possible to have someone keeping track of who comes and goes from a country's sky fleet, coming up to 'retired' members and offering them a chance to sail again. A great deal of crime is all about spying.
Piracy has generally been less about wealth and more about freedom and doing what you love but under no man's iron fist. For, most, that would be sailing the skies and taking jobs that they prefer. (The crew of Serenity in Firefly are effectively pirates for this reason.) Back in the golden age of piracy, more raids were about what was immediately needed and not about wealth. With the industrial revolution, I can imagine this changing to some degree, but not entirely.
I remember hearing or reading about a story where one pirate guy got paid a huge frigging diamond. Everybody else got lots of little ones. Incensed, he had it broken apart because he saw everyone else with more diamonds. The big rock alone was worth far more than any handful of the smaller ones.
What's sad is that even with training and specialization, the vast majority of people of the early 1900s were still as uneducated as those in the 16, 17, or 1800s. So, I can imagine history repeating itself.
