Unlike zinc, aluminum protects well in both salt and brackish waters, and even offers some protection in freshwater.
Aluminum is lighter, works better and lasts up to 50% longer than zinc in saltwater. Check current pricing. You may find that aluminum anodes are also less expensive than zinc anodes.
Aluminum makes the best “general purpose” anode for vessels which frequently travel back and forth between fresh and salt waters
For "freshwater only" applications, magnesium anodes are the only type that offer good protection.