For a change, this is a rather recent story. I have read it about 10 years ago. I think it might have been a bit older than that, but did not have this “taste” of being from the 80’s or earlier.
There are several point-of-view characters, and one of them is a woman. I don’t remember much, except that one of the characters (probably one of the point-of-view characters, but I am not 100% sure) was a spy. In fact, he did his spying on an civilisation of mostly underwater vaguely humanoid creatures, who could survive at least a bit out of water. The spy had some device that could make him amphibious (whether it make him look identical to the creatures he was spying on, or more human-like than them, but still amphibious, I forgot). How he first got hold of this device I also forgot, but when the creatures get on the ship and catch him, one of them tells the woman that the device itself was not so very important. What they really were after were all the maps he drew (or at least the things he remembered) about their resources, the strengths and weaknesses of their defences, and so on, to prepare for a invasion by his employers.
This spy story was only one of the threads of this complicated novel. I forgot most of it. The only thing I remember clearly is that when the water creatures boarded the ship to catch the spy, the woman tried to appease one of them by telling him (her? it?) about the spy's device. But though the creature tells her the device is not so important he/she/it adds that it still is "puissant", though the book was in english. The use of this french word (which means "powerful") to describe a device that still was considered as not so important is the clearest memory I have of the whole book.