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From what I gather from the Wikipedia page, it's due to the transmission cables being underwater and hence having a much larger capacitance, which is a larger loss than normal DC loss. (Correct me if I'm wrong on that.)

So, while AC is still more beneficial for transmission via cables in air/ground, DC appears to have less overall loss for underwater transmission.

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By virtue of their tight bundling, all composite multi-core cables have high capacitances. However, onshore AC transmission is typically run on overhead lines, with large line spacings and thus capacitance isn't as much of a problem.

DC is always more efficent to transfer. But it was difficult to efficiently convert to high voltage and back so you only did it when necessary.

The reason underwater links (eg France-UK) were commonly DC is that they linked countries whose grid wasn't synchronised - the same reason long distance grids in the US are DC

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