Personally I love the fact that the Merpeople are capable of fighting back on par with a navy ship/fishing boat. It's so unusual for the 'victim class' being oppressed to actually get their act together and devastate their enemies.
I floved Decker's response to the admiral trying to shift blame by saying "WE didn't underestimate YOU DID." total mic drop moment.
Honestly I'm confused by the Afro American mermen and mermaids since from an evolutionary point of view those merpeople should be albinos if they live in the ocean with little to no light. Oh well I'm just going to pretend that a Afro American man knocked up a mermaid and she laid a whole bunch of eggs. I'm really hoping we get some more mermen action. All this polyamorous femaleness is so 90's, bring on some polyamorous mermen to the party and this might seem more sexually progressive than the average lesbian teasing stuff.
As for possible racial subtext, we've seen 3 merpeople and other than that time Rin was trying to murder Ben in the water that one time, they've had pretty good reason to attack humans, 'Donna' murdered a whole bunch of people that had imprisoned and 'experimented' on her, Merman was attacking the people he thought were involved with starving and trying to capture other merpeople and Rin murdered one guy who was trying to rape her. Frankly all the merpeople violence seems pretty well justified to me so I don't see any negative racial subtext.