Well, the current understanding is that the World pre-Flood was much warmer than present, like a greenhouse. All that decay in the core kept the Earth nice and warm. The reason why it wasn’t too warm was due to the large subterranean ocean deep in the crust. Just as how the oceans today help regulate the Earth’s temperature, these massive bodies of water helped do this too.
Problem is, super-continents are terrible when it comes to maintaining planet temperature (in secular models, they always break apart shortly after they form because they become too warm). So when the Flood started, the heat from the radioactive core caused the supercontinent to fracture. And this was helped by several asteroids. Shock Dynamic Theory believes that a large impact just North of the Seychelles several million years ago fractured the tectonic plate there, and changed the direction of continental drift. This is more than likely caused by the Shiva bolide, which was the big brother of the Chicxulub and Bolytsh bolides (whether these specific impacts came earlier or later isn’t too important; these show there was colossal asteroid bombardment).
With the splitting of the Earth’s crust, the Fountains of the deep opened. And all that hot water came up (though the video I put in an earlier comment explains what happened during the Flood itself better). All the rich ecosystems were destroyed and mass buried, and in those warmer conditions under all that warm water and above the warmer land; they would become fossil fuels (there’s a nice report on how the Kentucky coal beds were formed from a massive log mat during a large flood event). These large bodies of hot water would have formed hypercanes, creating large-scale rainfall. Thanks to the two different bodies of water’s temperatures’, such hot water wouldn’t have necessarily mixed with the surrounding cooler water (like a bubble). This would form controlled regions of hot water only a short distance away from their source, with the hypercanes only forming above them to quickly spread the heat and water away (that’s why hurricanes form after all, to transfer heat from the Equator).
With the entire planet covered in water, heat was lost VERY quickly (volcanic activity was still ongoing, there would still be plenty of ash blotting out the sun). The planet cooled, and slowly the energy required to maintain the ferocity of tectonic activity it was undergoing was lost. This was also helped by the rapid subduction of water into the mantle along with entire tectonic plates (hence the large cold slabs in the mantle near the core, and the fact that magma can only exist in the presence of water; and magma had been found deep within the Earth. The sheer amount of heat lost into space would’ve been incredible.
The more water taken into the mantle, the more the cooling. And this cooling helped set up the resulting ice age which lowered the sea levels. The sea level rise we’re currently experiencing will never be as high as the Flood was, because a lot of that water is too deep within the Earth. And the uplifted land wasn’t present before.