Generally speaking URNU Summer Balls tend to be black tie.
As has already been said you can check with your host, you will probably have 3 options, all of which are acceptable if male:
1) Mess Undress(Black-tie, bum-freezer Mess Undress, miniature medals)
2) Destroyer Rig (Best uniform with bow-tie & full-size medals)
3) Civilian black-tie.
If a girly, additional options:
1. Ball Gown (But most definitely check that others will be dressed similarly or you will most definitely stand out!)
2. Below-knee cocktail dress, usually shoulders covered by a shawl. (RM's usually opt for this one too!)