Type 316 stainless steel is often the default choice for corrosion resistance in seawater. However, in hot seawater (e.g. desalination plants) or under high structural loads, it does not perform as well as 904L, being more prone to pitting and cracking. Type 316 is also commonly used in chlorinated water and in swimming pool hardware, where it has better corrosion resistance than 904L. Type 316 is prone to cracking if in a corrosive environment under under heavy loads, such as architectural structures.
Note that the above mention of "high loads" indicates stresses that would immediately destroy a human arm: for use in watch cases, the difference in performance will be apparent only in circumstances that kill the wearer. If you expect to come out alive, do not concern yourself with the strength and corrosion resistance of the grade of stainless steel.
You could look up performance data on stress corrosion cracking and PREN for austenitic and superaustenitic stainless steels in engineering handbooks or American Society for Metals data sheets, but you won't find anything more authoritative than Rolex advertisements.