I used to think that too, was taught it fact.
Experience has proven my teachers to be wrong in so many things. - like the idea that the 300 series is all non magnetic... try it, you'll be surprised too.
Like I said, SS is a big subject - but as general rule 316 is the most corosion resistant and the only one I would consider "Marine grade" (even though it will still rust - they all will).
"Good marine hardware made of 304"?
Just last week I had to throw away my boarding ladder - 304 and rusted out from the inside - Couldn't buy one in 316, so had to build one.
"If I knew what I was talking about"... cheap shot - You got shares in a chandlery or something?
OK, so what you're really asking for is for others to validate and agree with your opinions -- you could have saved a lot of time if you had just labeled your thread that way to start with.
BTW -- my guess is that your "304" ladder either wasn't 304 or that it had other issues going on inside it.