Fish do not travel in their bowl full of water; sloshing water stresses fish and is a spill risk in a moving car. Bagging the fish in a portion of its own tank water is the same technique pet stores use to send fish home.
Pro tip: A battery-powered air stone dropped in the bag is worth it for trips over an hour, especially in warm weather.
Watch out: A fish left in direct sun inside a sealed bag can overheat within 20 to 30 minutes, even on a mild day.