I've shown deniers here how longwave IR warms the oceans before, even though it can barely penetrate into the oceans. They always respond by flinging insults, running, and then just repeating their debunked crank conspiracies again. I'll do it again now, so we can all watch them meltdown and run again.
(Yes, I do understand that since this is high-school level stuff, most of the deniers won't be capable of understanding it.)
This is the temperature profile of most spots in the ocean. Note that the vertical scale is sort of logarithmic.
The bulk of solar energy penetrates deeply and warms the water. Convection causes warmer water to rise, so the ocean temperature gets higher as you get shallower.
However, that trend reverses at the skin layer. The atmosphere is usually colder than the ocean below, so the ocean at the surface loses heat to the cooler atmosphere. That lowers the temperature of the surface.
The amount of heat flowing out the oceans (from both conduction and evaporation) depends on the delta-T across that skin layer. Heat conducts from hot to cold, linearly proportionally to the temperature difference. With more of a temperature gradient, more heat flows out of the oceans. With less of a gradient, less heat flows out.
Enter the additional longwave IR radiation. It heats the skin layer, decreasing the delta-T across the skin layer. That means less heat flows out of the oceans. The IR doesn't heat the deeper ocean directly. It reduces the heat flow out of the deeper ocean, so more heat stays in the deeper ocean, so the IR indirectly warms the deeper ocean.
That just warms the surface waters, however. How does the heat get down way deep? Thermohaline circulation. Look at up. At certain spots in the oceans, the surface waters dive down deep. That's mainly what carries heat down into the depths.
