On a gut feel basis, I'd say your budget is light. Living on $500 a month means you're basically living in poverty...perhaps with a view. Also, dont forget to budget in regulatory fees and such. Entry/exit, visas, registration, etc.
Also, without insurance, you can likely forget about staying at a marina as a minimum of liability insurance is usually required to stay for any duration of time.
Can you save up another $100k and then go? $15k a year seems like its a lot more doable than $10k a year. Also, 5% return on AVERAGE means you'll have years where you'll lose money. If you lose money early in your investing cycle...your LONG TERM real returns will dramatically be lower.
Now, that being said...why do you need to preserve principal? If you're able to get 5% return and then also extract from the kitty, sure you may not have much at the end of your cruise, but you can work for the rest of your life!
Here's a couple that did it and their costs:
I'm a big fan of the way they did it...but you can see that they had some means.