large turtle, sea turtle
黿 centers on a large aquatic turtle, specifically a sea turtle. The two given meanings are essentially the same animal described in different contexts.
黿 combines 元 (origin, head) on top with 黽 (frog, toad, or a general amphibian/reptile) on the bottom. The character likely represents a large turtle or sea turtle, with 元 possibly contributing a phonetic element or suggesting a prominent head.
The top part 元 looks like a round head or shell, and the bottom 黽 is a creature with legs and a tail. Together they form a large turtle: 黿.
For ゲン, imagine a giant sea turtle named Gen: Gen -> ゲン, and the turtle's huge shell is the origin (元) of its strength.