Originally a compound of Old Japanese elements 痣 (aza, “bruise”) + 等 (ra, a pluralizer, “lots of”) + 獣 (shi, “beast, especially one used for meat”). The bruise meaning was in reference to the spotted coat.
The 海豹 spelling is an example of ateji. The characters literally mean “sea leopard”.
Alternative forms 
海豹 (hiragana あざらし, katakana アザラシ, romaji azarashi)
- an earless seal, a true seal (the mammal), a member of family Phocidae
Usage notes 
As with many terms used in biology, this is often spelled in katakana, as アザラシ.
See also 
海豹 (traditional and simplified, measure word 隻/只, Pinyin hǎibào)
- seal; earless seal; true seal