Discussion from 2010 suggests the wall is still there, and unbelievers are on a timer before they're cemented into the wall of the faithless.
Quoth:
It's still there until they specifically say it's not. We've been told to assume that unless something is described as different or gone, then it's still there.
Details of the wall:
Some minimal amount of real worship is required to initiate a patron relationship with a god. Without a patron, no god can claim you, so you are stuck in the Fugue. The servitors of any god who is not your patron cannot even see you, speak to you or interact with you in anyway. After a time, if no god comes to collect you, you feel the siren tug of Kelemvor's city calling to you to come for judgment.
The faithless begin to petrify and are tossed onto the wall. There is a moss that acts like a mortar that is corrosive and breaks down the substance of the petitioner body. Eventually the petitioner body is eaten away.
Moreover, there seems to be discussion of discussion of the wall in 4e fiction:
The wall was also mentioned in 4e novels (Edge of Chaos briefly mentions it IIRC)
On the other hand, heroes of shadow has worked out the Keeper of the Everflow ED. While the Heroes of X books are certainly not FR, the wall of the faithless can trivially equate to the raven queen stealing souls for her afterlife and Keepers can exist trying to restore the "natural" order of things.
Traveling by sea might have been really dangerous - not more than traveling by land in certain areas - mostly because of living hazards.
Crews that do not pay homage and make sacrifices to Umberlee, or that are not protected by Valkur or Shaundakul, might be ambushed by creatures of the deep, and the same goes for enemies of the Society of the Kraken.
Some regions are also controlled by the typical D&D sea civilizations, including kuo-toa and sahuagins, and pirates are surely a thing, but land travel has orcs, goblins, kobolds, the more rural giants (ogres, trolls and hill giants) and bandits of all sorts. The only reason why traveling by sea might be more dengerous is that the ships are mostly used by land-dwellers and their guards are not really good at patrolling the sea as they do with the high roads. While this is also true for the real world, the real world balances that by having the main source of piracy among land-dwellers as well. D&D has underwater races that are way more accustomed to the environment. Basically, ships have the same protection as conestoga caravans crossing the wild west. There are hostile Indians, there are pacific Indians who won't go out of their business to protect them and there is the rare fort here and there (guard ships): I'd expect ships to travel in groups and with armed escort. Especially convoys who want to travel to the hostile, unknown coasts of Matzica to look for the riches Amn is importing already.
Coastal travel is way safer because there are civilized cities on the coast and partolling the coast would be expected.
Ships from large merchantile companies are expected to board some kind of priest of mage that who cast control weather or generate winds on command, making weather a minor inconvenience.
The spells and equipment published in It's wet outs- uh, I mean Stormwrack make the whole seafaring thing safer. Sextants do exista and they "only" cost 250 gp (and even without one, using stars to find the north is just a survival check). Control currents, favorable wind and the everfull sails make it hard to be hampered by a calm, while detect ships and flowsight protect from being approached by pirates and sea creatures.
Casting locate city from Races of Destiny means being able to find nearby ports. As always, casters are really great at helping even outside of combat.
Of course, adventurers more easily sail towards the danger than not, but I'm pretty sure crews might hire adventurers to protect their charge. How many adventurers are ok with this kind of life depends, I guess, on how much they get paid.
Best Answer
Seawater is colloquially used to mean saltwater. While there are a few named seas that aren't saltwater and oceanographers do not use sea scientifically - most seas are saltwater. What makes a body of water salt versus fresh water comes down to 3 factors: how big is the body, how old is the body, and how many tributaries enter the body.
The Sea of Fallen Stars has many tributaries, is old, and is very big.
Additionally, while it isn't explicitly stated, much of the material for life in the Sea of Fallen Stars designates if it is fresh or salt water life.
http://www.realmshelps.net/faerun/lore/ecologies/fallenstars2.shtml#top
(Originally from Elminster's Ecologies)
Typically when a setting is written, anything out of the ordinary is stated. Often things that aren't stated can reasonably be assumed to conform with the most common versions of those things. We don't describe what legs are when we say a creature is a biped, for example.
So given these 7 pieces of evidence: that people often mean saltwater when they say sea, that none of the material seems to state that it is freshwater, the assumption that most seas are saltwater, that the sea is big, has many tributaries and is very old, and that life is described in the area as being both salt and fresh water - I'd say that it is very likely that the Sea of Fallen Stars is intended to be saltwater.