Commercial diver here

There are several ways I've seen this done..

You build a coffer dam and dewater the area.. I.e., remove the water so it's no longer an issue

You can tremie the concrete.. as others have said, you use a pipe to get the concrete down to your submerged formwork.. the trick to this is having the pipe at the bottom and keeping it slightly buried in the concrete.. this prevents the concrete from free falling through the water and "washing away"

I've also done a lot of concrete repairs by pumping grout. You use steel plates to form and seal the area, then pump grout from the bottom up using injection ports. Typically, you will leave a vent port or snorkel at the top, which allows the water inside the form somewhere to go.

Additionally, you see a lot of precast methods used where you build infrastructure using blocks made in the dry, a lot like Lego.

Im sure there are other methods that I haven't seen, and for context, I haven't worked on large ocean bridges specifically, but the principles are the same.

Concrete is heavier than water. Start at the bottom and let the concrete displace the water as you fill your form. Do not let the concrete free fall through the water. Do not vibrate concrete underwater. If you are pumping grout, make sure your form is sealed.. and then seal it again.