The primary use/purpose of water in light water reactors is neutron flux moderation and cooling/steam generation. Water is only used for shielding during re-fueling. In land based PWR's (pressurized water reactors) the thickness of water to effectively protect workers from the radiation in the core when it is open for refueling is around 23 feet. Basically there's a 23 foot swimming pool (don't go swimming in one) between the fuel rods and the workers.
Shielding with various metals of different thicknesses is much more economical for a ship and a power plant. While there is a lot of water available at sea, there's no need to drag around enough water inside the ship to shield people from the radiation. You may as well sink the boat. Metal shielding is the way to go.
Also consider that nuclear powered naval ships do not often refuel, maybe once every 20 years or never. Since the core will not be opened, no need to provide a fluid barrier.