I know that models of ocean planets typically have deep liquid water that transitions into exotic forms of ice. What I am wondering is if it makes sense to have an "ocean floor" composed of debris from bombarding asteroids during the early formation stages?
Super-pressurized ice probably wouldn't be static. It would (almost certainly) move around and shift with the seasons and planetary climate changes and all that good stuff, so I'm extremely doubtful that anything would stay "littered" on the floor in this scenario.
Other factors would be whether the planet had life, which can drop shells or scales and other stuff, which falls to the ocean floor, and volcanic activity which likely happens on most planets and would eject some matter to cover whatever was on the surface over time.
Plate tectonics, obviously another factor though I'm not sure how plate tectonics would affect a thick layer of ice.
This planet is too hot for life (probably) and likely too large for plate tectonics, but I'm sure the hot ice still shifts and moves around and the interior heat has to escape from time to time, either cracks in the surface or volcanoes. I think you'd need a nearly dead surface like the moon or mars for things to "collect" like that. I don't think an exotic ice planet would cooperate in that way.
what I am trying to do is come up with a good scenario where life "as we know it" could arise and flourish on an ocean planet. To me that meant needing some sort of ocean floor that wasn't just some exotic form of ice. I've seen standard models of ocean planet interiors, but hadn't really considered that there will be a wide variety (ie. the interior of Ganymede is probably quite different than Europa). So, in the right conditions I suppose volcanism and plate tectonics may be the answer (super-earths may be more tectonically active than Earth)
Lots to cover here, I'll give it a shot briefly.
Life may have arisen on earth near the ocean floor but not on the ocean floor. it was the hot vents of gas that Extremophiles lived in. I don't think they require the ocean surface, just the volcanic jets with methane and other gasses that they could use. (I think they use Methane, not 100% sure). I think they require liquid water, but not ground.
Since the deep below the surface of planets tends to be hot, exotic ice would usually be quite hot, I would think, just solid under very high pressure, I suspect that only Extremophiles could live near exotic ice in those high temperatures. High pressure is less of an obstacle but high temperature might limit the options for life to the very primitive/tiny (I think). The good news is, nearer the surface, you might get lower temperatures and that might open the door to more advanced life. I think what's on the surface is generally more interesting than what's on the floor. Ganymede and Europa both have frozen surfaces with occasional breaches from tidal forces. A larger planet might have exotic ice deep down but more earth like oceans towards the surface. I think that would be a lot more interesting to us.
I don't know how many varieties of water worlds there are. Neptune is a water world of sorts though it's usually categorized as an ice giant and it probably doesn't have what we would consider oceans but it has a lot of water.
On, Plate Tectonics, it might not be essential to life on water worlds. Volcanism can exist without plate tectonics and Volcanism is probably essential to life on water worlds. Don't get me wrong. Nothing beats plate tectonics for circulation and recycling of materials and preventing unwanted buildup of waste, so plate tectonics is really cool and better than just volcanism, but I think active Volcanism would be sufficient for life in oceans. It's really hard to say anything definite about this subject though, as it's based on theory.
It's probably possible to have volcanic eruptions even though dozens or maybe even hundreds of miles of exotic ice because the heat has to go somewhere, eventually, assing it's likely to build up over time, so either by circulation of eruption, the heat has push through at some point. This even happens on so called "dead" planets like Mars or even the Moon. Mars still has the occasional volcanic eruption, just not very often.
But water worlds certainly can have plate tectonics. There's nothing in the water that would prevent it from happening. Plate Tectonics is, as I understand it, primarily a factor of the size of the planet. Gas planets - different story, but planets with a hard surface, Earth sized, a tiny bit smaller to a fair bit but not much bigger are good candidates for plate tectonics (I think). There's some debate on how large, I think, still going on. But I remember reading that ocean/water worlds might even be more likely to have plate tectonics. Plate tectonics is definitely something we'd look for if we ever get a close enough look at other planets in different solar-systems (exoplanets).
Just my thoughts on this. Not meant to be complete or definitive.
You get an ocean planet because in the early phase of formation it was bombarded with ice. But everything is relative including the composition of asteroids and planets which will accrete more or less based on the composition of the primordial disk from which they are formed. Some ocean planets will have more ocean and less to no rock while others will be mostly rock with a few miles of ocean overlaying the hard parts like Earth. Your proposed scenario is a good one except keep in mind that what we are dealing with with that much water is a whole bunch of a substance famous for being the "universal solvent"! So whatever solid material crashes into the ocean of that planet that isn't vaporized by the impact would still probably end up in solution or suspension. Have you ever microwaved a cup of water and then put in a spoonful of sugar?