Half the Day Is Night — Maureen F. McHugh

War veteran David Dai has come to ocean-bottom Caribe to work as bodyguard to Mayla Ling, banker and scion of the undersea city's old-money set.

But as Mayla negotiates the biggest deal of her life, she draws the attention of terrorists who threaten to plunge her and David; back into the nightmare of his violent past.

Evoking a fully realized 21st century with the astonishing depth and realism that is McHugh's hallmark, Halt the Day is Night is a further advance from one of the best new SF writers of this generation.