Assassin’s Creed Odyssey has 42 Underwater Locations for the Child of Poseidon Achievement. You must complete all of the underwater locations by finding the sunken treasures in them. Use your eagle to mark the underwater treasures. Be sure to clear out the location so it shows a checkmark on the map or else it won’t count.
Locations map can be found here:
Underwater Locations 1-5
1 - Ktesipo’s Shipwreck
2 - Islet of Zeus
3 - Poseidon’s Fury
4 - Sunken Temple of Demeter
5 - Ketos Shipwreck
Underwater Locations 6-10
6- Alekto Shipwreck
7 - Ruins of Arne
8 - Persian Remains
9 - Purrhos the Red
10 - Underwater Cavern
Underwater Locations 11-15
11 - Sunken Temple of Aphrodite
12 - Smuggler’s Shipwreck
13 - Stormfall Bay
14 - Shark Bay
15 - Prototype Trireme of Ameinokles
Underwater Locations 16-20
16 - Aruabignes Shipwreck
17 - Phoibean Lagoon
18 - Phaleron Sunken Harbor
19 - Vouliagmeni Sinkhole
20 - Underwater Cavern
Underwater Locations 21-25
21 - Submerged Minoan Palace
22 - Aigisthos Palace
23 - Shipwreck
24 - Nereides Monument
25 - Olympic Shipwreck
Underwater Locations 26-30
26 - Underwater Ruins of Dyspontion
27 - Shipwreck of Nestor
28 - Dorion Underwater Cave
29 - Pavlopetri
30 - Sunken Wreck of Datis
Underwater Locations 31-35
31 - Ruins of Seas People
32 - Shipwreck of Ajax
33 - Palace of Amphitrite
34 - Anaphi Shipwreck
35 - Sunken Temple of Eileithyia
Underwater Locations 36-40
36 - Cave of Wealth
37 - Sunken Ruins
38 - Shipwreck Cove
39 - Hermit’s Dive
40 - Polydektes Lagoon
Underwater Locations 41-42
41 - Wreckage of the Lost Bandit
42 - Shipwreck of Dionysos and the Pirate
Solution sourced from PowerPyx:
http://www.powerpyx.com/assassins-creed-odyssey-all-underwat...