[RPG] Handling many similar opponents in combat

Yes.

From the DMG 3.5, page 93:

Characters swimming, floating, or treading water on the surface, or wading in water at least chest deep, have improved cover (+8 bonus to AC, +4 bonus on Reflex saves) from opponents on land. Landbound opponents who have freedom of movement effects ignore this cover when making melee attacks against targets in the water. A completely submerged creature has total cover against opponents on land unless those opponents have freedom of movement effects.

(the same rule in the Pathfinder Reference Document)

My own interpretation of this rule is as follows:

If you are attacking with a weapon and you aren't subject to freedom of movement, then the surface of the water blocks line of effect. This means that two creatures who are at least chest deep in water have improved cover relative to each other. A creature who is completely submerged has total cover and therefore cannot be attacked.

PHB 3.5, page 152:

If you don’t have line of effect to your target he is considered to have total cover from you. You can’t make an attack against a target that has total cover.

You can Focus Power once per Round

Per the Dark Heresy 1st Edition errata, currently located at https://images-cdn.fantasyflightgames.com/ffg_content/dark-heresy/pdf/darkheresy-errata-v3.0.pdf

The first paragraph in the Using Psychic Powers section starting on page 161 should include the addition: “Making a Focus Power Action is the psychic equivalent of a Standard Attack Action, and counts as such for purposes of determining what else a psyker can do in a round. Therefore, a psyker who uses a Half Action power cannot make another Half Action Standard Attack on the same round. A Psyker may manifest only one ability per Round with the exception of the Resist Possession power. A Psyker is required to roll at least one die to manifest an ability.”

So no matter what, you can only ever perform the Focus Power Action once per turn, as it counts as a Half Action Attack (even if the power isn't an attack), and there are already rules in the Actions section regarding the inability to perform multiple Attack Actions (don't have my book to properly cite this, but the errata clearly states only one Action per Round).

The change in the errata was made to prevent Psykers from using both Focus Power and Half Action Standard Attack Actions in the same turn. The Action section in the combat chapter of the core rulebook (specifically under Half Actions) should have some sort of wording that states you can't perform the same Half Action twice in a round. You can't Standard Attack twice for example. You technically are taking a separate Full Round Move Action, not performing two Half Action Moves.