I am aware there are certain opinions of the flood that assert it was local, and that it was only 15 cubits deep (24.5 ft deep)
All sorts of thoughts and statements follow these assertions.
Was it really covering the mountains?
That would require several miles of depth, with the ark floating on top in the clouds.
Which is true?
It's a little more complicated. According to some experts, the pre-flood topology was much different. They stated that the highest mountain were much lower that they are now. Everest is 8,849 metres high. It's hard to imagine Everest being underwater. Yet marine fossils have been found on the highest mountain peaks.
We do not know the humidity in those days, so it is not possible to demonstrate that there was enough water in the atmosphere to cover the mountains of the time. However, The Bible talks about the fountains of the deep. They were opened up and seem to have been under enormous pressure. That was the source of the vast amount of water required to cover the earth.
The enormous pressure on the earth's surface caused weak points to collapse, allowing the flood waters to drain away. The Grand Canyon is an example. There are marine fossils on the top layer of the canyon, indicating that it was sea floor at one time. As the water drained into what is now ocean, it cut a path through the ocean floor, draining the water away. You can see a similar effect in soil erosion after heavy rain. Evolution says it took millions of years. Not really.
The same pressures forced relatively low mountains to rise up to where they are today. If that sounds far fetched, the earth is constantly moving even now. Everest in rising about half a metre each century.
The Canyon consists of layers of sediments. This was supposedly laid down by many floods over millennia. Yet there is no vegetation or animal fossils between the layers. If the layers had been there for thousands of years, there would be signs of life during that time. It was shown by the Mt St Helen's eruption that sediment layers can form in a very short period of time. You can simulate this in a clear container. Fill it with a dirt/water mixture and stir well. It will settle in to layers.
None of this can be proven. Neither can evolutionists prove otherwise. All they can do is cast doubt on God's Word. That's Satan's MO. Don't fall for the lies.