Here is the full list of all 38 Under The Waves achievements.
Find closure
Complete Stan's seventeenth day underwater
Complete Stan's sixteenth day underwater
Complete Stan's fifteenth day underwater
Pick up 10 empty oxygen sticks
Reach a score of 2500 on the punching bag
Complete Stan's fourteenth day underwater
Complete all photography missions
Take a peek at the surface
Use a repair kit on Moon
Complete Stan's third day underwater
Pick up 50 pieces of plastic
Pick up 25 pieces of plastic
Have Stan sing a sea shanty aboard Moon
Learn more about a blue whale
Complete Stan's second day underwater
Watch all optional TV programs
Find a collectible in a wreck
Interact with the communication antenna
Complete Stan's first day underwater
Unlock all achievements
Find solace
Reach a score of 5000 on the guitar
Play with the music box
Find the galleon
Find all collectibles
Unlock all stickers
Clean 50 oil traces
Find all crafting recipes
Craft every upgrade
Craft 25 oxygen sticks
Break Moon beyond repair
Succumb to ?????
Rescue a squid from a wreck
Upgrade Moon's engines to electric
Craft every decoration
Complete all routine missions
Unlock 75 logbook entries