Outside the Lamplight Security base, by the large rocks (to the left when exiting the vehicle spawn pad) there is a large field of "No Damage" that also counts towards underwater travel. It allows guns to fire but does not register damage on targets if the shooter is within the boundaries of the field. Conversely, firing at targets within the field while standing outside of it allows the shooter to inflict damage on those targets. Underwater classed weapons can inflict damage when firing from within the field. Here is a video from a buddy with us testing out the area of effect (language warning because I have a salty mouth):
