They do lose water continuously but replace it by drinking sea water and then desalting it. The salt is returned to the sea by active transport at the gills. Living in constant danger of dehydration by the hypertonic sea, there is no reason to pump out large amounts of nephric filtrate at the glomerulus. The less water placed in the tubules, the less that has to be reabsorbed. With a reduction in the filtration-reabsorption mechanism, the marine bony fishes rely more on tubular secretion for eliminating excess or waste solutes.
Tubular secretion requires a good blood supply to the tubules.
Lacking efficient glomeruli, the renal portal system must carry most of the burden.