Back in the day.........we had to make do with taillights that flooded when backed into the water. With jockeying around on the ramp, the brake lights were on a lot, which heated those glass bulbs up to the point where they would burn you easily. That hot bulb would smoke immediately as soon as it was submeged in the much cooler water - leading to the advice regarding unplugging prior to backing in.
Today's tail lights seal MUCH better, and often house LED's rather than bulbs, so they're much less critical when it comes to being submerged.
