We can approach deep sea locations and lower equipment (or people) with relative ease. A base at this depth would likely need pressure regulation, which is a lot cost to still have people working in atmospheric diving suits. The most likely candidate for a deep sea base would be a deep sea mining operation. I see no benefit for geology or biology research.

Meanwhile, Mars is not easy to approach. It might make sense to have staff who can maintain equipment and such. Besides, having people there is a political goal for the sake of it, while staffed deep sea bases are rarely talked about.