I’m not a geologist, but I’m imagine that the deep ocean would be a colossal underwater glacier, with intermixed sedimentary layers. Kind of like what we have with methane hydrate deposits, only much, much deeper. The super-deep ocean simply wouldn’t exist, and we might not even know about the Mariana Trench, or a lot of other sea floor features. Also, it’s possible a different proportion of the world’s water would be frozen in this way.

With ice as a part of the sea floor, it would also interact with subduction zones at continental edges. That might push a LOT more superheated water into volcanoes, faults, and everywhere else water could go. That would probably make for a lot more geysers in such areas, and volcanic eruptions would be far more energetic.

The trajectory of human history and technology would also be changed. There might have been fewer ice bridges between continents during the last ice age. Ice-skating wouldn’t become a thing. Harvesting ice in the winter would require bodies of water to freeze solid first, making it impractical except in shallow areas.

I’m also going to wager that glaciers would behave differently too. I don’t know enough about their dynamics, but I wonder if having meltwater on the bottom helps lubricate their movements somewhat. Kind of like a lava flow, only slower. Inverting that relationship might make glaciers far less mobile.