We cruised for six years and it's a problem for many. We knew a couple of cruisers who were trying to do it with investments -- except they weren't doing long term investments, they were trying to do market timing. That's a problem when internet connections are spotty. Others did canvas work, deliveries, under the table diesel or outboard work, etc. -- but nothing that was a reliable source of income. Some had rentals "back home" -- and that was a problem with property management.
Others periodically went home and worked as temps -- particularly in the medical fields. Knew several nurses and one PA that did this -- it worked well, except for taking a year out of cruising and finding a place to leave the boat.
The bottom line is that there isn't a great way to earn money as you cruise -- some people have some skills that give them some extra money but few have something that works well as a steady income. Still, an extra $100 or $200 a month can go a long ways if you don't have a big boat payment or feel the need to stay in high-end marinas, etc.
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