Please remember that we live in one of the hottest states in the summer! Outdoor summer weddings are beautiful, but they are HOT! A couple of cute wash bins full of ice and water bottles is a nice thing to do for your guests.
Put them by the guest book, by the ceremony location, wherever you can. If a guest arrives 20 minutes before to get a good seat and the ceremony is about 30 minutes long, they will be sitting out in the hot heat for almost an hour before the reception starts. So buy a lot - you won't waste it! The alternative could be guests passing out due to the extreme heat.
An alternative to water it a cute lemonade stand or even lemonade favors! It is a great way to put your unique spin on the favors while replenishing your guests' thirst!