Hi @alctraz99 ,
Its maybe an old game, but still a lot of fun to be made with it !
Unfortunately there are also (still) some old and known bugs in it, whereof quite many were left behind unfixed.
The one you mention from the underwater diving in the campaign chapter is one of those.
There was no solid workaround found for it. It is merely random appearing. Experienced it also myself twice.
Only thing to do was to quit the campaign. Then first try and play some normal gaming, go and try a segment of another Campaign chapter. And then go back again to the Campaign chapter on the boat where you need to do the diving. And then try again. I tend to recall also on the PS4 platform that some said you could and should just keep swimming and swimming and you would come through underwater to exit point, so it was like everything suddenly moved in slowmotion. Think there also was a part involving you to jolt the joystick repeatedly for the duration of the underwater swim, for the soldier to move forward.
There is another known bug of similar type, when you get through the chapter with the Locker area, and you get outside the mountain edge and you have to jump across the scaffold. If you are unlucky, then it will never recognize that you made that successful jump, so you are eternally stuck there. Only way to "proceed" is to quit out of the Campaign environment and go back in and restart the Campaign and that chapter/segment.
Best of luck soldier!