I was completely wrong : ignore this, delete it :P.
Solution : the Action menu, underwater, allows you to switch to underwater/dual ammo. Don't know why I did not see it in the showcase. I see it when testing in the Eden Editor.
[ IGNORE / DELETE FOLLOWING :
I did the underwater showcase.
I randomly did a reload before entering water, so the game inserted a normal STANAG magazine (30x 5.56 bullets).
Using the normal bullets, I never hit any of the two underwater enemies that sometimes jump from the ship. No matter how many times I tried, even at point blank, they did not seem even hit once.
BUG : I was incapable of switching to the underwater (dual) magazines. Don't anyone dare tell me this is by design. Reloading automatically rotated only between STANAG magazines. Going to the water surface does not allow accessing the Inventory either, so. Having to get on land to manually, in the Inventory, force the use of the underwater (Dual) magazines is not by design. I am saying this in advance as people will naturally try to find some logic. You can switch STANAG magazines, so there is no logic of not being able to insert the proper underwater magazine.
I can accept the locked Inventory (by some logic I do not know), but players should not be stuck with the wrong ammo underwater, until they hit the ground to fix it.
If there is a normal way to fix this : would like to hear it.
One abnormal way I found was to get close to a dead body and use the Action menu to "Resupply 5.56 Dual ammo". Of course, I had to go on land, to do the manual Inventory switch described above, before I could ever see any dead bodies underwater. :P
I set this bug to Major as it is gamebreaking for any player stuck with the bad ammo underwater. ]