You could increase the buoyancy by something like 230 kilograms per cubic meter if you fully saturate it on a hot day, but I suppose you'd have to dam up both sides to keep the brine from washing away.
At this scale, even air pressure would be a factor, I'd presume a higher air pressure would be more conducive as large surface area of the water than the ship, though might be wrong.
Would that level of salinity be harmful once it reaches the sea?
Assuming you narrowly dam up the canal near the ship to contain the brine, no I don't think so. Seawater is 3% saline and saturated brine is 26% saline, so you'd probably have sufficient mixing within the canal itself once you open the dams. The canal is 120 miles long and I can't imagine you'd dam up more than a third of a mile at maximum. But it would take a whole hell of a lot of salt.
Yes since the salinity transfer from the canal into the Mediterranean Sea is already enough to make species from the Red Sea colonize the east Mediterranean. The Aswan dam reduced further the amount of freshwater coming to the area. So I would guess adding more salt wouldn’t be too good for the eastern Mediterranean.