It's a flightless bird
As others have pointed out, a creature would not dive if it was unable to do so due to being too buoyant, and if it had no reason to.
As for reasons for diving, one is to get food and another is to avoid attack from aerial predators. The latter reason is taken care of by the armour, but we have to ask why a surface-dwelling organism would evolve armour instead of evolving the ability to dive, which is probably the easier solution for most organisms.
This is where buoyancy comes in. If the animal is very lightweight it will be hard for it to evolve diving behaviour, and it might take the armour strategy instead. Buoyancy means low density, i.e. light weight for the size. But why would an animal be so light weight? One obvious reason is flight.
So: we have to imagine that this organism's ancestors were birds, or pterodactyls or similar flying creatures. Think of something like a large sea bird, though not necessarily a whale-sized one. It eats something that lives near the surface, so it doesn't need to dive very deep (if at all), and it spends a lot of its time resting on the water surface, using its feet to paddle like a duck or a swan.
If food is very plentiful and predators are few, then birds will often lose the ability to fly. We just have to imagine that this happens here, resulting in a bird-like animal that spends all of its time paddling in the water, never taking to the air.
(One difficulty is that a bird would need to lay its eggs on dry ground, but if this is not an Earth species we can imagine that it has live young instead. Or it could evolve a special body cavity in which to incubate its eggs or something - you'll have to be inventive here.)
After that, we just need it to evolve its large size and armoured back. Sea animals seem to evolve a large size quite often. The reasons for this are not completely understood (though see here), but the same thing might well happen to a flightless sea bird. It would be easier for it to stay warm, it would have bigger fat reserves, and it would be so big that no predator could eat it. However, since it's sitting helpless in the water it could still be attacked by other aerial predators, who peck at its flesh without killing it. An armoured skin would be a sensible evolutionary response to this.