It is the displacement of the vessels weight and the freeboard remaining of a vessel that determines whether a vessel will float and or sink. A ship will float higher on salt water (salt water is denser) than if on fresh water (less dense).
A boat will float sideways if the water is too shallow and it touches bottom. Boats will float higher in water with greater salt content. If you fill a boat with water it begins to sink deeper. It all depends on whether you are talking about water outside the boat or water inside the boat.
A boat will sink until the weight of the volume of water pushed aside by the hull is equal to the weight of the boat. Salt water has slightly greater density(weighs more by volume) than fresh water, so a boat will float higher(push less water away) in salt water.