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The Nanaue (there is no wiki link) is a Hawaiian creature of myth. It is a shark with arms and legs. It can live in both the ocean and on the land. So how do I create the monstrosity that is, the land shark?
A list of all of the Anatomically Correct questions can be found here
Seagong crocodiles exist, and there was a period where crocodilians were rapidly evolving to fill multiple vacant niches (dinosaurs and mammals just were better at it, so we won). So theoretically, an advanced saltwater crocodile might have the adaptations you want.
The problem is that evolution favours adaptations that make the creature better able to function in a particular environment. A seagoing crocodile will become more like a seal or a real shark (notice the one who's legs have turned into paddles), while land going ones get longer legs, more ground clearance and beginning to look like dogs or even velociraptors. These one's are not going to do at all well in the water. Modern crocodiles are the way they are because the shallow waters near the edge of bodies of water are a specialized environment that they have adapted to so well the basic crocodilian body plan has survived since the Jurrasic period.
Evolution does not favour the creation of a natural "land shark" however.
The way I see it, you have two options.
Option one, darwinism/evolution.
Or, you could go with an alligator. Alligators are similar to sharks, and they are amphibious. Perhaps these alligators adapted to be able to swim in ocean water, instead of freshwater. Then they discovered their love for human flesh... Anyway, I think this would be a more likely prospect.
There are already 'fish' adaptations that meet this description (so reptiles and amphibians need not be involved). They are just not all present in a single fish/shark species at the same time, or to the same extremes in the same fish species.
Here is a shark that can already "live" out of the water
Here is a fish that can REALLY last a long time out of water (though, admitedly not a 'shark' specifically)
This fish has legs (again, not a shark, but still...)
There have been evolutionary pressures to cause not just any fish, but sharks specifically, to be able to leave the water, at least temporarily. There have been evolutionary pressures for fish to last a long time out of water, and grow what are functionally (if not by definition) legs.
It's not a big leap to think a shark could combine these, along with increased size, and that would match the description very well, especially if the "arms and legs" description is taken no more literally than absolutely necessary.
For the purposes of this, sharks don't have skeletons! The Nanaue would need one to hold itself up on land.
Then there's the problem of it having gills instead of lungs. I don't know of any creatures that breath above and below water.
The Nanaue could have evolved from a maniraptor that became semiaquatic. They might lose their feathers to reduce drag, similar to a seal. A mutation could lead to higher incidence of dicephalic parapagus twins. This would increase drag, and so they might evolve to have the heads fused into one. Their tail might increase in size, and gain a vertical tail fluke. In order to carry the larger tail, they may start to stand upright. Their arms and legs might become shorter, as they are less needed for swimming. Due to the gap in the back between the spinal columns, they might evolve to be able to snap the spines shut, to scare away predators. They may evolve sharpened ribs in this area, which would likely lead to the spines splitting as a normal part of their anatomy. They might evolve a layer of skin inside the spine-slit. This may evolve to evert as a dorsal fin. They might gain inflatable disply structures over their body. These might split from the respiratory system, to avoid the display structures filling with stagnant air. This could lead to them becoming able to be filled with water. The inflated form might become streamlined, with a pointed, shark-like head, and flipper-like arms and legs. They would evolve to pump in water rapidly to inflate their body to avoid deflation. Due to not needing to be skeletally streamlined, they might evolve to have a larger brain, and to become more intelligent. Their hands might become more dextrous to allow better tool use, until they appear relatively human in appearance. This creature would be quite like a Nanaue
My thought is that the epaulette sharks were suddenly introduced to a bigger predator and had to use their ability to flee the water much more often and eventually evolve longer, stronger fins and spend more time on land than water. They would still not look like human limbs or walk upright but they couldn't do that without millions of years or genetic engineering, which probably couldn't make that big of a change and survive
