Dive Operations    

US federal law 46 USC 2101 defines the term "recreational vessel".   Submersibles that do not carry passengers for hire fall into this category.   It is important to note that even if you carry one passenger for hire, your submersible is not a "recreational vessel" under US Federal Law.   The term "for hire" means that you are receiving some form of consideration (cash, trade, food, etc) in return for carrying one or more passengers within your submersible.


U.S. Code
TITLE 46, Subtitle II, Part A, CHAPTER 21
2101. General definitions

(25) "recreational vessel" means a vessel -
     (A) being manufactured or operated primarily for pleasure; or
     (B) leased, rented, or chartered to another for the latters pleasure.


References:
46 USC 2101