webfoot, web
蹼 centers on the webbed structure of a foot, specifically a webfoot or the web between toes.
蹼 combines 足 (foot) with 菐, which likely contributes the sound and may relate to a spreading or flat shape, together suggesting a webbed foot.
The foot radical 足 on the left and the spreading shape of 菐 on the right suggest a foot with skin stretched between the toes, like a duck's webfoot.
For ボク, imagine a duck's webfoot making a soft 'bock' sound as it paddles: bock -> ボク, and the foot is webbed.