I think you'll find that mangroves get most of their fluids from their food and use their salt glands to get rid of the excess salt from water taken in accidentally while feeding. Not many terrestrial vertebrates actually drink salt water intentionally.

As for being the only monitor to catch fish in deep water, Merten's water monitors would have them beat hands down in that area.

Basinboa is correct in that the species you've chosen isn't an ideal one for calm temperament, although there are calm individuals out there. However, be careful judging temperament from seeing videos/images on the internet of animals owned by others, as often listlessness resulting from poor husbandry is mistaken for calm temperament.