Here is the full list of all 9 Guns & Fishes achievements.
Die 1 time.
Get 100 headshots.
Kill 250 enemies.
Complete a level without taking any damage.
Kill 5 enemies using the octopus weapon.
Kill 5 enemies at once using a puffer fish.
Shoot 5000 bullets.
Kill 5 enemies using the squid weapon.
Reach level 20.