H₂O

Water vapor is already a crucial greenhouse gas on Earth, responsible for as much as half of its absorption of sunlight. And you've got tons of water to work with from the evaporation of your planet's oceans. While one might think an ocean planet might develop a thick cloud layer that reflects a significant amount of sunlight, cloud formation relies on condensation nuclei of dust and other particulates, and without any land to create dust there might be a bit less cloud cover. Then the extremely humid, but not-quite-condensing, atmosphere can do its part to warm the planet.

CO₂

And I don't think aerobic bacteria would be enough to form a suffiently CO2 heavy atmosphere.

This is backwards. Carbon dioxide readily formed in any cool enough environment during the solar system's formation, and it's relatively nonreactive so it tends to stay on any planet with enough gravity to hold onto it (hence we see lots on Mars and Venus). Billions of years ago, Earth's atmosphere was rich with CO₂, and only after photosynthesizing organisms emerged did oxygen mostly replace it. Since you're asking how to dramatically warm this planet, I must assume it's quite far from its star — so photosynthesis is fairly weak and perhaps what plant life exists hasn't been enough to drop CO₂ levels to near-zero (though hopefully it's low enough that people can breathe there).

Albedo

Not a greenhouse gas, but this'll help a bit — oceans are dark and generally absorb more light than landmasses. So even if your planet was otherwise identical to Earth, one would expect it to be slightly warmer per the blackbody equations.

H₂O

Water vapor is already a crucial greenhouse gas on Earth, responsible for as much as half of its absorption of sunlight. And you've got tons of water to work with from the evaporation of your planet's oceans. While one might think an ocean planet might develop a thick cloud layer that reflects a significant amount of sunlight, cloud formation relies on condensation nuclei of dust and other particulates, and without any land to create dust I'm not sure if there would actually be any more cloud cover. Then the extremely humid, but not-quite-condensing, atmosphere can do its part to warm the planet.

CO₂

And I don't think aerobic bacteria would be enough to form a suffiently CO2 heavy atmosphere.

This is backwards. Carbon dioxide readily formed in any cool enough environment during the solar system's formation, and it's relatively nonreactive so it tends to stay on any planet with enough gravity to hold onto it (hence we see lots on Mars and Venus). Billions of years ago, Earth's atmosphere was rich with CO₂, and only after photosynthesizing organisms emerged did oxygen mostly replace it. Since you're asking how to dramatically warm this planet, I must assume it's quite far from its star — so photosynthesis is fairly weak and perhaps what plant life exists hasn't been enough to drop CO₂ levels to near-zero (though hopefully it's low enough that people can breathe there).

Albedo

Not a greenhouse gas, but this'll help a bit — oceans are dark and generally absorb more light than landmasses. So even if your planet was otherwise identical to Earth, one would expect it to be slightly warmer per the blackbody equations.