I have two of the David Boye Dendritic Cobalt folders; a serrated sheepsfoot and a partially-serrated "pointed" or leaf-shaped blade. Both have a folding marlin spike on the handle. If you're mostly going to be cutting rope, I would recommend the sheepsfoot. The pointed version is better for all-around use. My pointed version felt dull out of the box, especially the plain-edge section, but with a sharpening, it performs great, especially on fibrous materials. For cutting rope, either Boye folder is awesome. And the serrations are not going to snag, because they're ground smooth and shallow. The titanium marlin spike is great for loosening overly-tight knots, some very light prying and, say, for opening boxes of food.
On one of my Boye folders, the marlin spike is extremely tight and difficult to open; on the other it is smooth and easy to access. The action on either of my Boye folders is not as crisp as on my Spyderco Salts. The lockup on my Boye sheepsfoot has some slight blade play, but nothing that negatively affects the usage. I would guess that the lockback mechanisms on the Salts are probably stronger than on the Boye knives.
Either the Boye folders or Spyderco Salt lineup would fit the bill. Interestingly enough, this isn't the first recent thread started by someone seeking a saltwater folder who says they don't like Spyderco knives/the Salt lineup. Otherwise, I would have recommended one of the Caribbean models.
Jim