It's revealed that the creatures cannot swim. However, this doesn't preclude them from stalking just below the water, which we see in the basement during the first movie.
The corpse of Emmett's wife that Marcus discovers is mummified and decayed, but mostly intact. It makes sense that nothing would have disturbed her remains, given that most scavengers would have been killed or driven off by the monsters, and none of them eat what they kill.
It's unsurprising that oxygen tanks would be left behind for Evelyn to find at the pharmacy that she visits for supplies in this movie. They are clunky, awkward metal cylinders, and most people who needed them would have perished in the beginning of the invasion.
Emmet’s idea to save Regan by simply making noise so the monsters will go after the bandits is brilliant because it shows why most people don’t try to rob or attack each other in this world, all anybody has to do is make some noise to attract the creatures and their attackers will be forced to worry about their own survival and can no longer be a threat.
It's also why the ambush of the dock bandits likely succeeded for so long. Most of their victims would rather try to survive without their possessions than risk inciting a monster attack by resisting or fighting back.
It's possible that the dock bandits knew that the island over the sea was a refuge from the creatures, and used this to their advantage - to ambush desperate survivors who attempted to flee to the island using the ships that were tethered on the docks. What if they were turned away from that island because of their wickedness, and decided to stage their robbery there partly to take their revenge?
It's heavily implied that at least the ringleader of the dock bandits is interested in molesting Regan after he separates her from Emmett. The implications of being kidnapped and sexually abused in a world of monsters lurking around every corner are horrifying. How many other young girls has he taken advantage of in this manner? Is the feral girl who lures Emmett into their trap just one of his victims who has accepted her lot as the bait? Thankfully, he got what he deserved, but how many other such evil people are still out there?
The fact that the aliens can't swim, but can drift onto land on an empty boat, is a troubling thought. Not even an island is 100% safe from the monsters in this world. What if some are smart enough to realize that they can sneak aboard ships and hold off on murdering their crew long enough to attack populations that were previously unreachable?
We know the aliens can't swim, but we don't know for sure that they can drown. For all we know, there are creatures wandering on the ocean floor, looking for whatever they can kill, or for a way back on land.
