Here's a rant on water's influence on combat [spoiler] I prefer a model where water negates most projectiles. Sure a grenade won't stop from exploding, but most of its forward momentum will be absorbed. Bullets momentum is also absorbed by water, and they often break apart because of the force. Water would stop the forward momentum of plasma "bolts," and cool them, although it might make a pretty boiling effect on the surface of the water. Being a few inches under water would certainly make you nearly invincible, for a time. Although, that's only true for a certain distance under water. That distance might be up to 16 inches, but its means that that model can't work for that distance. In that case, I would agree with reduced damage, proportional to the length of water the projectile must traverse to hit its target, perhaps offset by amount of armor. Grenades though would be different. Their arc would change once it hits the surface of the water, but the force of its energy would still create a deadly pressure wave. The fragmentation that is the hallmark of modern grenades might be completely negated, or have a reduced radius. But the pressure wave would be deadly farther than it would be in the air. What difference this makes with the grenades damage radius is hard to say. The effects could negate each other, and they behave functionally identical although with difference causes of death, or the radius could increase or decrease. It should probably decrease, as the deadly radius of a grenade (in the air) from its pressure wave would be very small, while the radius for its fragmentation is quite large (in air), and while the pressure waves deadly radius might increase in the water, it would still not likely compare to the fragmentation radius of a grenade in the air. Hard to say for non-fragmentation grenades, although generally water would absorb a lot of energy. In order for me to argue about waters effect on space magic, we would need a physics equivalent. Is a Nova bomb a literal piece of plasma, or is that just a way to describe it. If the former, water would absorb it, if the latter, its anybodies guess. [/spoiler] TL;DR Water will absorb the energy of most projectiles. After a sufficient depth (up to 2 ft), most projectiles will cause no damage. Grenades will likely have a reduced damage radius, and of course will move differently in water than in the air. Space magic [i]probably[/i] behaves similarly to plasma, so it would be absorbed by water too, although there is no way to know, and Bungie can do whatever they want here, physics be damned.

I think you covered almost everything. Acid rain might be cool.