This novel won the John W Campbell Memorial Award for 1986, which is certainly a mainstream SF credential, and it was eventually followed by three more novels comprising the Elysium Chronicles. As The Encyclopedia of Science Fiction describes it: "The planet (in fact a moon) is in this case a water-covered Utopia inhabited solely by parthenogenetic web-footed aquatic female Shorans ... whose pacific culture suffers a savage Invasion at the hands of the male-dominated rigidly-hierarchical culture from the neighbouring planet of Valedon, whose leader is called the Patriarch." Sloncewski is apparently a Quaker, and her pacifist beliefs are definitely explored in this novel. What's interesting is that the Patriarch is light years away from and thus invisible to the planets he rules through an intermediary called the Envoy, and so he works as a kind of metaphor for the Christian God that the Quakers also worship. Perhaps Sloncewski's willingness to explore her conflicted feelings about her religion is part of why the novel feels so personal and honest, despite the way it stacks the moral deck in favor of the Utopian female society.
Like a lot of '70s feminist Utopias (cf Russ' Whileaway, Charnas' Motherlines, or Piercy's Woman on the Edge of Time) Slonczewski's matriarchy reflects the political division within the womens movements of the time, with more militant factions, some that are more spiritual, and some that are more separatist, for example. Nobody gets let off easily, and a lot of the novel is taken up with the anguish various characters (including the militarist males) feel about the decisions they make. The deep history of the novel seems to be that the Shorans are descendants of people they call the Primes who thousands of years ago destroyed their civilization with "fire," probably of a nuclear nature. This forced them to change their culture and their science. Now perhaps the Patriarch is interested in resurrecting that old technology for his own purposes, or whatever other weapon technology the descendants of the Primes are capable of creating with the new science.
As Jones implies, what makes A Door into Ocean particularly fascinating is that the women of the ocean moon Shora are advanced genetic scientists, using only organic means to manipulate genes and cells. Slonczewski's background as a microbiologist shines through in the marine ecology she creates on Shora, where all life forms are interdependent and healing is practiced through enzymes and specially-bred lifeforms rather than pharmaceuticals and scalpels. (The split is reminiscent of Sterling's Shaper/Mech stories, I guess.) It's a work of hard science, and a highly original one. The only similar worldbuilding I can think of is in Varley's Eight Worlds stories and Gaia trilogy. Slonczewski goes much deeper, to my mind, creating a fully-imagined world that brought me that vicarious pleasure of exploring the alien that I remember from my adolescent encounters with science fiction. The sexuality in the novel is interesting too, with the one relationship that we see most closely being between one of the merwomen and a male (or malefreak, as the all-female Shorans think of him) from the planet that is invading their world. They are biologically incapable of having reproductive sex (in fact they are basically toxic to each other on that level), but the sex they are able to have is smoking hot.
Despite the painful subject of genocide and resistance, there is a joy to this novel that is a pleasure to behold: a science fiction writer in her prime hitting on all cylinders of imagination and speculation. Great stuff, highly recommended.