Fish
* Barracuda
* Bass (many varieties, like Striped Bass, Sea Bass)
* Blenny
* Bluefish
* Bonito
* Butterflyfish
Other
* Barnacle
* Beakhead (a type of squid)
* Beluga whale
* Bigeye tuna
* Box jellyfish
* Brittlestar
Bonus
* Blacktip reef shark
This list is not exhaustive! There are many more fascinating creatures in the sea that begin with "B."