St. Brendan's, Traveling mailbox, and many others work very similarly, in that they're all designed for those on the move. It's not just boaters, but RV'ers, traveling nurses, even some pilots and flight attendants.
Now, where they start to differ is in whether they can also serve as a physical address and their use on other things like state of residency. You need to decide your objectives up front. Do you want to remain a resident of the state in which you currently live or another.
St. Brendan's is very popular because it's in a state with no state income tax, because it is recognized as a physical address and you can get driver's license and all other needed items with it as your address, because it has an excellent system for processing your mail and because of their knowledge and experience in receiving and forwarding packages to you out of the country and minimizing custom's issues.
If using a relative you face a few issues. First, do you want them to see all your mail. Second, do they have the ability and willingness to scan. We have a family member pick up all of our mail from our PO Box and home while we travel. They do exactly what St. Brendan's does and that is scanning and then destroying or holding or forwarding as we desire. If it's an envelope they aren't certain if we'd want them to open, they scan it first and ask.
Now, psneeld's problem was based on ignorance in the post office and not making a proper distinction. If you're using a CRMA (Commercial Mail Receiving Agency) such as Mailboxes, etc or UPS store or others, then the Postal service will not forward mail from it to your new address. But if it's a marina or business or anything other than a CRMA, then they will. The key isn't business but it's CRMA.
That said, forwarding from a marina gets complicated because you're depending on them picking your mail out of all mail addressed to that location. So, it's important to have a specific address there. If you're having mail just sent to your attention at Marina ABC then forwarding later will be of questionable dependability. If though you were Marina ABC, Slip 24, then it should work better. The problem then is only that the Post Office doesn't have that set up as a separate location and full zip code. But there is nothing keeping them from doing so. A Marina slip is no different than an apartment number. Personally I would never use a marina unless they had a row of mailboxes like an apartment complex. Otherwise, they are one address, your mail in with all others, and depending on them to handle your mail as you need, might be asking too much. Legal items mailed to you are considered delivered when mailed, whether you get them or not.
So with a marina, I'd get a local PO Box for mail and use the marina only as my physical address.
Every state is different as to becoming a legal resident. Florida is easy, although you still need to validate that you're taking steps in your previous state to become a non-resident there. We were legal residents in FL in less than a week from the time we decided to become so.
