I will preface this post by acknowledging that Florida's firearms statutes are confusing and present ambiguities. This post is not legal advice, but rather my best interpretation of this legal issue as of the date of this post, though I will do my best to update it.
The fishing, camping, or hunting exception
Most people in Florida cannot carry a gun openly such that other people can see it, unless the hunting, fishing, and camping exception applies. "A person engaged in fishing, camping, or lawful hunting or going to or returning from a fishing, camping, or lawful hunting expedition" is allowed to open carry their weapon and need not have a permit to carry their weapon concealed. Fla. Stat. Section 790.25(3)(h). (There are some other exceptions in Section 790.25(3)).
But I interpret Section 790 to nevertheless require secure encasement in your vehicle, because the section that addresses private conveyances is subsection (5), and it must be separate for a reason.
Visible but still securely encased inside a vehicle
Florida statutes section 790.25 describes lawful circumstances for carrying guns. It says it is legal for a person traveling in a private vehicle to possess a firearm when it "is securely encased." Fla. Stat. Section 790.25(3)(l), (5). You cannot carry a gun on your person in a vehicle without a concealed weapons permit. Fla. Stat. Section 790.25(5).
"Open carry" means someone can see your gun, but it must still be securely encased inside your vehicle. Thus, carrying a firearm inside your vehicle is not so much about whether the gun can be seen, but rather whether it is securely encased. This would permit a firearm to be in a secured holster, say with a hammer strap, to comply with the law even though the gun can be seen from outside the vehicle.
Otherwise, it seems a gun must be (1) concealed if the person has a permit to carry concealed and (2) securely encased or otherwise not within ready reach, if the person does not have a permit to carry concealed.
Castle doctrine extended to vehicles to a limited extent
In one's own home or business, a person does not need a permit to carry concealed. In one's own home, a person may also carry a weapon openly. Fla. Stat. Section 790.25(3)(n). The law does not require secure encasement in one's home or business.
However, rules for possessing a gun in a vehicle are covered under a separate section. Section 790.25 does not list one's vehicle as a place where a person may carry a firearm openly.
Florida law's overall preference for guns hidden from view
Our laws' general preference is for guns to not be on display except under certain circumstances. There are several places in Section 790 that express our legislature's preference for guns to be hidden from view. For example, business owners cannot condition employment or discriminate against customers based on whether they have guns in their car provided that the gun is "out of sight" inside the person's vehicle. Fla. Stat. Section 790.25(d).
The case law addressing the criminal prohibition on carrying guns in a vehicle does address what amounts to "concealing" a gun in a car. For example, a gun sitting on a passenger seat does not violate the law, unless it is covered up by something. In Hinkle v. State, 970 So. 2d 433 (Fla. 4th DCA 2007), a gun covered by a bouquet of flowers on Mother's Day was considered a violation of the carrying concealed firearm statute. Such cases imply that that keeping a gun on the passenger's seat would not violate the law against prohibition on carrying a concealed firearm if not for the gun being covered up.
Handguns are treated differently
I interpret Section 790.25 to mean that the carrying of a handgun inside a private vehicle is more restricted than that of, say, a rifle. The rationale for this is probably to avoid restricting carrying of firearms commonly used for hunting:
(5) POSSESSION IN PRIVATE CONVEYANCE.—Notwithstanding subsection (2), it is lawful and is not a violation of s. 790.01 for a person 18 years of age or older to possess a concealed firearm or other weapon for self-defense or other lawful purpose within the interior of a private conveyance, without a license, if the firearm or other weapon is securely encased or is otherwise not readily accessible for immediate use. Nothing herein contained prohibits the carrying of a legal firearm other than a handgun anywhere in a private conveyance when such firearm is being carried for a lawful use. Nothing herein contained shall be construed to authorize the carrying of a concealed firearm or other weapon on the person. This subsection shall be liberally construed in favor of the lawful use, ownership, and possession of firearms and other weapons, including lawful self-defense as provided in s. 776.012.
Less privacy in a vehicle
Remember that you have less privacy in a vehicle than in your home. So you can expect that an officer who sees a gun in your car sitting on the passenger seat will seize it for safety purposes, at the very least. Ensor v. State, 403 So. 2d 349 (Fla. 1891). This seizure could lead to an arrest for something other than carrying a concealed weapon. Some examples would be for the firearm having an altered serial number, or for possession of drugs that may be found under or near the firearm the officer seizes.
You can't control how your gun possession is interpreted by others
A fellow motorist who gets a glimpse of your firearm while traveling may have a negative reaction to it. Diffusing rather than instigating such reactions should be your goal, in my opinion.
Americans are severely divided on this issue. But remember that police will also react to your firearm possession, and rightfully so. Their safety is threatened often and they will not abide a firearm in a vehicle during even a routine traffic stop without insisting that the firearm be handed over, at least temporarily. Pollack v. State, 600 So. 2d 1313 (Fla. 3d DCA 1992).
In Conclusion, just because you can do something, doesn't mean you should. You carry a firearm in public at your own peril in so many ways. Carrying a gun in the most secure, safe way in any given situation is the best practice.