Last night I created a stub on Wikipedia for ocean suffocation, and it has survived at least to the time of writing this.
The purpose was to help highlight for non-specialists a phenomenon that may prove to be important. As noted here, Ralph Keeling has suggested 'ocean oxygen deprivation'. But it's not clear to me that 'ocean suffocation' is any less accurate or more sensational than, for example 'ocean acidification', which is now a widely accepted term.
Anyway, this needs attention from people better-trained and wiser than me.
P.S. 24 Oct: After discussion involving distinguished oceanographers and others the wiki stub is now labeled ocean deoxygenation.