I've seen salt water ponds before. There is a koi place near here who converted one of their stock tanks over to a salt pond and he has coral, fish, inverts, the works in there...
the pond has to have a roof over it otherwize as you stated the salinty would drop to low the other issue you will have is heating unless you decide to keep coldwater animals, i currently have outdoor salt pond it has a roof over and is 4000liters i use 3 300 watt heaters 24/7 in winter to keep water at 23% cel . my electricity bill is horrendous