They do regulate salts. They tend to take up water from the sea which they use for excretion. They drink sea water and absorb it into the body. Ammonia is lost through the gills and very little excretion is needed so that little fluid is lost via kidneys. The highly evolved pipe fish and sea horse have aglomerular kidneys and do not filter blood in the kidney.
All excretion is handled by cellular pumps and diffusion.
They cry very salty tears as you know from the tales of Alice in Wonderland and the Walrus and the Oysters. Marine mammals have very efficient kidneys with long Loops of Henle that can concentrate urine to osmolalities higher than sea water. Most also eat fish which have a lower salt content. They also conserve water when nursing by producing milk devoid of sugars. Their milk is high in fat which is a good calorie source and does not contribute to osmotic pressure.