Preheat the oven to 375°. In a small saucepan, boil 1/2 cup of the balsamic vinegar until reduced to 3 tablespoons, about 5 minutes. Let the reduction cool, then stir in 1 tablespoon of the canola oil.
Heat a grill pan. Grill the corn over high heat, turning, until nicely charred all over, about 5 minutes. Let cool. Using a serrated knife, cut the kernels from the cobs.
Arrange the salmon fillets skin side down on a work surface. Using the tip of a sharp knife, make five 1/2-inch-deep slits crosswise in the flesh of each fillet. Stuff 1 slice of ginger into each slit, then thread each fillet lengthwise with a short skewer.
In a large, ovenproof, nonstick skillet, heat 1 tablespoon of the canola oil. Season the salmon fillets with salt and pepper and add the fillets to the skillet, skin side down. Cook over moderately high heat until the skin is lightly golden, about 3 minutes. Transfer the skillet to the oven and roast the salmon for about 6 minutes, turning once halfway through, until the skin is very crisp and the fish is just cooked through.
In a large bowl, whisk the remaining 2 tablespoons of balsamic vinegar with the mustard and the honey. Gradually whisk the remaining 1/4 cup of canola oil into the dressing. Add the grilled corn kernels and watercress to the bowl and toss to coat. Transfer the salad to plates and top with the skewered salmon. Drizzle the fish and salad with the balsamic vinegar reduction and serve right away.
To make a larger batch of the reduction, start with at least one cup of vinegar and boil until syrupy and reduced by more than half. Refrigerate without the oil for up to 2 weeks. Drizzle the balsamic reduction over grilled vegetables or fresh fruit, or add to sparkling water. Variation: Stuff the salmon with small slices of prosciutto instead of ginger.