You are correct in what you write; oceans are absorbing majority of heat from a warming planet. This is having all sorts of impacts on marine ecosystems (infact, this aspect is really one of the forgotten areas of climate change, somewhat 'out of sight, out of mind'). To your point, a warming ocean will impact submarine operations - that is a dimension that this film would explore, particularly in the Arctic. Interestingly, one security expert we spoke with mentioned that the melting Arctic and break up of surface ice is impacting operations right now (whereas once they could use the 'cover of ice', this is no longer feasible).

In regards to expanding thermal ocean. This is one of the contributions to sea level rise. It is especially pronounced in the tropics for obvious reasons. From memory - and I would need to fact check - but it contributes something like 20% of sea level rise (or something like that). The majority, of course, being from melt-water of glaciers / ice and so on. All told, rising sea levels are a major global challenge. this includes militaries that have vast infrastructure and other bases that will be (and are being) impacted by sea level rise.