From an official State of Florida government web site:
Frequently Asked Questions
"1.What are the requirements to become a resident of Florida?
Florida does not have rules for residency in general; residency in Florida is program specific. That is, it is attached to a specific purpose or need, such as taxes, in-state tuition, etc. ..."
Do you get it? "Residency", in general legal terms, is determined by where you physically park your butt and hang your hat. Where you either reside, currently, or have definite plans to return to after sojourning elsewhere.
The fact that Florida, unlike many other states, has no definition or requirements for residency does not make your SBI mailing address into a "residency". The fact that the Florida DMV will accept that mailing address, without using the USPS database and throwing it out for being a mailstop, does not make it a residency. And their DMV's own rules make that not-quite-clear by the actual way they refer to the address- -in their own requirements--and not to your actual physical residency.
As usual, Florida hasn't quite "gotten on the program" with the rest of the Continental US.
What you have is a "residential address" for the sole and "program specific" purpose of a Florida driver's license, which is not the same as a bona fide residence or residency. That other agencies blindly accept this is very nice, but in the years since 9/11 we've all seen a number of agencies, including the Florida DMV, reverse themselves on matters of what is valid ID and what is residency. Heck, the states can't even agree on whether they can or will issue driver's licenses to trespassers and felons, i.e. illegal immigrants.
By all means, as long as the SBI mailstop works for your needs, enjoy it. Just be aware that will probably change,
and if that's all you've got when it inevitably does change, you may be caught in a tight spot.
Apparently, right now, your boat is your only and actual
residency. Even if Florida is kind enough to ignore that, the Florida's DMV is primitive enough to accept that. In most other states, a DMV would accept that as your mailing address--but still require a separate physical
residency address, which could even be "Aboard the good ship ..."