One thing to note is that heat diffuses much slower through the ocean than the atmosphere - even if the impact on the average temperature of the ocean is neglig

One thing to note is that heat diffuses much slower through the ocean than the atmosphere - even if the impact on the average temperature of the ocean is negligible, local thermal pollution could still cause major issues. I don't know how much effect a pod of this size will have; hopefully Microsoft will fund and publish a comprehensive review.