There are things above the clouds that drop aircraft out of the sky without survivors - telemetry tells us they are huge, gigantic creatures, reptilian, undoubtedly some form of Flying Dinosaur - but what they are doing above the clouds, and where they nest is unknown, and without a safe way of getting up there and physically looking (and coming back alive) its likely to remain a mystery.
The Oceans are similarly problematic - occasionally a vessel can travel relatively long distances without issue - other times, even the shortest of journeys can be fraught with danger - and ships, and crews - vanish without trace.
A Short list of possible encounters at Sea (and these aren't even the Obvious ones).
Jaekelopterus - A type of Sea "Scorpion" - imagine more than 8 feet of armored and clawed horror. Which could happily clamber on board a smaller boat. Originally they were extinct way before the Dinosaurs came along - nowadays they are swimming around side by side.
The Dunkleosteus was a 30+ foot long Carnivorous tank! Instead of teeth, it had a bony "beak" like arrangement - like a turtle. It has been calculated that they had a bite force of 8,000 pounds - enough to bite through the hull of a ship :/
Kronosaurus, a short-necked pliosaur, was a “mere” 30 feet long and the longest teeth in its massive mouth were 11 inches long. A Kronosaurs head was around 9 feet long- basically one could pretty much chew on a whole person in one go, and still have room left half another. Some Scientists have theorized that since their flippers really similar in design to those of modern sea turtles, that they may have crawled out onto land to lay its eggs (I would hate to be on the beach during breeding season).
And what with the Basilosaurus (imagine an 80+ foot long Carnivorous alligator/snake/whale that makes the Great White kinda pointless) my feet would be staying on dry-land.
If being out in the Barrens is scary in S.D.Z.A. - then being out on the Oceans would be terrifying!