Optimator is correct. If your boat is not registered in some other state (even if it is USCG documented), then technically speaking you are supposed to register it immediately upon arrival in Florida. Depending on the age and length of your boat (assuming it is not larger than 65') the cost to register could range from as little as less than $10 up to somewhat less than $200.
On the other hand, I have never heard of anyone being ticketed for this. It seems that, at least in general, the authorities realize that a strict interpretation of this law creates a hardship for many, and so it appears that they exercise some discretion in its application.
That is not to say that you should just ignore the law, or assume that you have 90 days. Technically speaking, you do not. What's more, given the relatively low cost of registering in Florida, it seems to me it would be prudent to register your boat at some early convenience. If it were me, though, I would not be overly concerned about IMMEDIATELY registering, the first moment I cross the border into Florida.
The issue is TAXES and whether such laws are enforced or not.
Generally, if a boat registered (sales/use taxes paid) in one's home state and then taken to another state and kept there for beyond
state's 'grace period' of 15, 30, 60 or 90 days, then the new
state will declare itself as the state of principal usage
and one then is subject to pay the differential (difference) between what the new state charges and the what the home state charges ... SALES/USE TAXES, that is. This applies not only to boats but also to automobiles, aircraft, trucks, buses, etc.
Overstay the 'grace period' and then interest and penalties can also apply. Some municipalities/counties in some states also apply personal property taxes, etc.
Florida additionally applies a 'sojourners fee' on boats after their 90 days grace period, even if the tax differential is zero.
Although such 'enforcement' may be lax, when traveling through various states one should have an onboard copy of previous taxes paid ..... AND
receipts (fuel, food, restaurant, lodging, etc.) that show exactly WHEN the 'vehicle' entered the new state's borders - starting the new 'tax clock'. Without such documentary evidence, the taxman will usually 'over estimate', and usually never
in the owner's favor.