The knife that rides on my surf belt is a Frost's of Sweden Swedish Army Knife... a Mora if you will.
I spend a week each fall fishing the beaches at Montauk.
The only thing that gets rinsed with fresh water each night is my rod & reel.
Waders, surf top, pliers and knife get maintained when the week is over.
Here is the knife after a week of being submerged in pounding waves, put away wet, & a long damp ride home in the cooler.
I took the photo right after I took it, and the rest of my gear, out of the cooler...
As you can see, it was still wet.
The little bit of rust is right where the drain hole in the plastic sheath is.
That little bit of rust came off with some 000 steel wool and WD40.
Would never know it was there.
If you are going to minimally maintain your knife any Stainless Steel will do fine (corrosion wise).
If you are willing to do daily maintenance on your knife you could use carbon steel.
I did for years when I boat fished with a Schrade Sharpfinger.
Get back with the boat, wash the boat, wash the rods and reels, clean the fish and wash the knife.
The sharpfinger was patina-ed like an old hammer but didn't rust except for once when I forgot about it for a couple of days.
(too much beer in the hot sun and not a fish to clean

)
The knife had a fine dusting of orange powder on it that came off with.... you guessed it... WD40 and 000 steel wool
In my experience you really have to neglect a knife to get it terribly rusted and even then it will sharpen up & cut stuff.