yes
Have never heard of an underwater assault rifle. In the US, you must be 18 to buy a rifle, and 21 to buy a fully automatic rifle.
Yes
The SKS is not an assault rifle, nor even a BATFE-defined "civilian assault weapon". And no, a permit is not required.
For a semiautomatic rifle, you must be 18. For a TRUE assault rifle, which is capable of burst or full auto fire, you must be 21. Federal law.
Assault rifle.
Yes
Yes you can own a SBR or any NFA weapon in florida.
The SOAR (Special Operations Assault Rifle).
Yes
There are several different assault rifles - both by the true definition of an assault rifle and by the political motivated BATFE definition of a so-called "civilian assault rifle", manufactured by several different outfits. Which assault rifle did you have in mind, specifically?
There is no assault rifle in Halo 2. They replaced it with the battle rifle which in it's own way is pretty good.
The first true assault rifle debuted in 1942.