Because the artist's fine with drawing Akagi as a seal, that's all.
Yeah, no pun in Japanese. Seal (animal) and seal (emblem, stamp, closure, etc.) sound nothing alike Japanese and share no common kanji.
Interesting side note: Seal (animal) is written in Japanese literally as "Sea leopard." Meanwhile we have the related but distinct animal we call a sea lion (which is written in Japanese as "sea donkey").