oceans flow east to west
https://en.wikipedia.org/wiki/Ocean_current#/media/File:Corrientes-oceanicas.png
On Earth this flow is interrupted by continents that form the great ocean gyres. There would be a flow induced by thermohaline pressure differences, that is in the poles water would cool and freeze out making it cold and salty, this would pull currents into the deep that would imitate the Great Conveyer
https://en.wikipedia.org/wiki/Thermohaline_circulation
But without the land masses messing it up.

The planet would also be circled by belts of winds, closer to the abstract 3 cell circulation models.

https://en.wikipedia.org/wiki/Atmospheric_circulation#/media/File:Earth_Global_Circulation_-_en.svg

These would affects surface current directions and thermohaline by evaporating some places and making water salty and raining other places and making its salt concentration drop.

So sort of how they work today without continents.

BUT the great huge steaming elephant in the room would be lack of CO2 sequestration from rock weathering. Spin up an Earth with a few exta kilometers of water to make it Water World and you wuold get huge build ups of CO2 over millions of years.

But here we go from a model running for a month to a model running for a couple of million years.