Sure can!
Water resistance is much, much higher than air resistance. And the faster the bullets are traveling, the more resistance they have, therefore the faster they slow down.
When I was in college, we did this as a fluid dynamics problem, modelling the bullets as perfect spheres travelling through still water. We found that a standard handgun bullet would slow to non-lethal velocities within about six feet. If someone is firing at an angle to the surface (which they usually would, getting six feet of water between yourself and the bullet would be entirely plausible.
But that’s just math. What about real life? In fact, firing guns into water tanks has long been an accepted way of collecting bullets for ballistics analysis. The water stops the bullets cold, without allowing them to collide with anything solid.
But what if you use a more powerful gun, with bigger and faster bullets? Well, as in so many things, the Mythbusters answered this one. Not only did they show that the six-foot calculation was actually pretty generous (the bullets they tested dropped below lethal velocity within about three feet), but the really powerful guns they tested were actually far LESS effective when fired into water. That’s because colliding with water as those speeds caused the bullets to fragment, as if they’d hit a solid wall. The fragments then dispersed in the water and lost their speed almost immediately.
So, some firearms might be more effective than others, but if you managed to get a few feet underwater when being shot at, you’d probably be okay. Certainly, if it were the only option available, it would be well worth trying.