Can typically be identified thanks to the attire's characteristics: black swim trunks covered by a blue cloak with golden embroidery.
May feature his weapon: a red book with a tomato on the cover.
The design of this outfit is identical to the swimsuit Leo wore in Fire Emblem Fatesโ Beach Brawl DLC. The Lilith-shaped floatie is also from that DLC.
Note that it's possible (especially for older artwork), to depict Leo in a swimsuit without it being the one described above, in which case this tag should not be used.