Volcanic eruptions have a cooling effect (they throw lots of small sulphur particles high into the atmosphere. See, eg, the Pinatubo eruption of 1991 caused a drop in average temperature of 0.6 C over a few years).
This one underwater threw a lot of water vapour up, which is a greenhouse gas, but one with a short life in atmosphere. The net effect would be minimal. The record heat is global warming as usual, coupled with a developing El Nino and the tapering off of the ability of the oceans to absorb excess heat. Think of it as a practice run for the next decade.
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