Here is the full list of all 24 SUBS achievements.
Reach level 5
Reach level 10
Reach level 15
Reach level 20
Reach level 25
Kill your own support helicopter
Visit the shop 5 times in one run
Kill an elite boss
Hit a helicopter out of the air with a boat
Equip 10 different upgrades in one run
Have 10 shark pets in one run
Fire torpedoes bigger than your submarine
Get a combo of 5x in Combo Rush
Reach max difficulty
Use a sea mine to kill a boat
Get a score of 200 in co-op
Get a score of 2000 in co-op
Have 100 ammo at one time
Witness lightning hit a helicopter
Go very high
Get a score of 2000
Enter the beam
Get caught up inside a tornado
Die