Sealand is usually depicted in a sailor's uniform, which is a white shirt with horizontally striped neck and cuffs, light blue shorts, and a large sailor's hat with horizontally-striped ribbon trim.
In the manga, Sealand also wore tube socks and Mary Jane shoes, but in the anime the socks were removed and the shoes changed to loafers. Furthermore, the necktie on Sealand's shirt was also less pronounced in the anime and the color scheme of the outfit darkened slightly.
These are all the outfits Sealand has worn. Please note that this list is incomplete. Please click for larger viewing.