Wet soils cause an oxygen deficiency, which reduces root respiration and growth. Root failure reduces nutrient uptake, and plants will eventually die unless drowned areas dry out quickly. A few days in waterlogged soil can be enough to kill young canola plants.
Excess moisture can reduce soil nitrogen levels through leaching and denitrification. A top dress could address this, but excess moisture and other weather factors that set back the crop may also reduce overall yield potential, which means the crop may not take up as much nutrient anyway. Make sure the crop is recovered and healthy before making any further investment.
What happens to canola plants that are underwater for a few days? Read a detailed explanation from Murray Hartman.