Be sure you have selected your spear gun/pneumatic spear gun as your active pole while in Sans Culpra.
Just above the "my profile" and "my crew" is your fish timer, which will display as "fish now!"' or list the time remaining. If you click the word "fish" (not "fish now"), you will toggle between fish mode and scuba fish mode.
You can be sure you have successfully switched as the header will now read, "scuba fish now!' or "scuba fish in __ minutes". Your oxygen tank meter will also be showing.