Engineer says No
What you're asking for is a mess of contradictory design constraints. Let's start with flying/swimming first.
There's a reason dolphins don't fly and (say) eagles don't swim. (1) Water is a much higher pressure environment, especially at depth. The flight capabilities of an eagle are achieved through increased lift and decreased weight. Eagles (and other birds) have lightweight bones that are hollow to decrease the amount of weight they have to get airborne. Their wings are large so that they can maximise the amount of lift they get on the air currents.
Those same wings (and especially the feathers) would create increased drag in the water that would make it problematic for swimming. Their large pectorals are great for flapping wings in the air, but simply can't compete with the hind muscles that power flukes and tails in the water on cetaceans and fish respectively.
Dolphins also need increased strength to swim and that is achieved through muscle mass, and generally strong bones that reinforce the structure of the animal at depth. This means that a dolphin the same size as an eagle would be denser and heavier, because they're bodies are tuned for different environments.
Now let's deal with scale strength. Ideally, you might be able to use some natural form of kevlar or carbon composite for the scales; this would give you strength without massive increases in weight, but there is a price. Both materials can be brittle, and kevlar in particular carries almost no resistance to heat, melting or cracking very easily. This means that your fire breathing dragon stands a very strong risk of harming himself with his breath, especially if it's powered by a high phosphorous content (allowing underwater fire). This would however give a credible reason why the scales would glow burning amber - the poor thing has actually set itself on fire.
Finally, riding. You're on a creature that is already heavy to support itself under water, but has traded off part of that weight for lightweight, combustible scales of natural kevlar, that breathes a phosphorous fire. IF it can fly, it would have to have massive wings, not only to support itself but to support you, which creates drag impediments under water.
In short, this animal is a danger to itself before anyone else, and if it CAN fly, it can only just do so. Want me to saddle up?
All I can say is you first.
(1) Yes, I know that dolphins can jump out of the water and eagles can dive for fish, but they don't spend extended periods in each others'domain for reasons set out above.