Objects (including boats) are more buoyant in salt water, than in fresh water because salt water is denser than fresh water. The salinity of the ocean varies, but the generally accepted average amount is 2.5%; so salt water weighs 2.5% more than the same volume of fresh water.
A cubic foot of fresh water weighs 62.4 pound (density of 1.0); whereas a cubic foot of salt water weighs about 64 pounds (density of 1.025) - So there is 1.6 pounds more buoyancy in a cubic foot of salt water than in fresh.
Due to the shape of a sailboat, the volumetric displacement increases slightly with immersion (as it sinks lower), so the actual difference in buoyancy will be less than 2.5% (only a symmetrical cube, with a consistent waterplane area, will feel a 2.5% difference between salt & fresh water.
See also Ted Brewer on POUNDS PER INCH IMMERSION (PPI): The weight required to sink the yacht one inch. It is calculated by multiplying the LWL area by 5.333 for sea water or 5.200 for fresh. The PPI usually increases as the hull sinks into the water as the LWL area is also increasing due to the shape of the hull above water.
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Ted Brewer Yacht Design Accordingly, I would expect less than a 1-1/3 inch increase in draft (between salt & fresh water), for your boat. Note: It’s not correct to say that salt water is more buoyant than fresh water. Objects (boats) in salt water are more buoyant than objects in fresh water. The buoyant force is exerted on a boat, not the water itself.