It is a formal military event.....so go dressy! A shirt and tie is appropriate....Look good for your Sailor! Business casual is fine even.
lot's of folks have asked: read thru this link: http://www.navydads.com/forum/topics/2150375:Topic:11793
To go along with what Tom said, your new sailor will be in their dress uniform. You gotta ask yourself how do you want to look next to your new very squared away sailor in all of the pictures that will be taken of the two of you together.