So let's talk a little about what kind of under water combat would be possible if we are allowed some in the near future.

Ballistic weapon damage is all but out at Myth Busters proved on their show (not that the game needs to be realistic or anything). I would think that lasers would also suffer badly with water fracturing the light of the laser everywhere as it tried to pass through. Plasma would explode as soon as it hit the water I believe. Gamma might work ... not sure how well radiation would pass through water like that.

So maybe it will just have to be the Harpoon gun, or do we actually have other options?

PS - would be cool to use a PA as a deep diving suit.

The Gauss Rifle would work, that's like a railgun right?

But yes, energy weapons wouldn't work along with ballistics. Gamma doesn't affect irradiated monsters so I can't imagine it being too effective.

Outside of including a harpoon gun or melee weapons, I'm don't know of any weapons that would work under water. I'm not familiar with "real world" weapons to know if there is another type of ranged weapon outside of a harpoon gun. Melee weapons would work, but reguardless of your perks, you weapon speed should be reduced/penalized beacause of the water's resistance. There must be some type of explosive that can be used under water, like C4 with a timer?

My only other thought would be that maybe the Junk Gun that you find might actually have some use. If it propels junk by using air pressure, perhaps it can be modified to shoot out junk using water pressure?

Ballistic weapons would work under water, just at SIGNIFICANTLY reduced range..... Damage falloff over range would be like a steep cliff. Gauss rifle may suffer somewhat less, as it start off with a much higher velocity. Energy weapons would be all but useless, at anything other than point blank. (barrel pressed against target.....) Rocket Launcher? Good question...... Grenades, and other AOE ballistic weapons would be useless as well.

A sound emitting weapon could work.

Some type of electricity weapon could be cool, assuming that we have some kind of electricity resistant armor.

A shock based explosive weapon (underwater grenade?) could be pretty effective and fun against organic enemies. Ripping their insides apart.

Why wouldn't energy weapons work? Laser weapons should work at the cost of accuracy and range assuming that they are waterproof. They should still be pretty ok short to medium range.

I used to kill sharks with what we called a bang stick (when I was much younger and crazy). Nothing like that in FO4 now though so they would have to add it. No range on the weapon but it was very effective.

Dolphins trained to attack with machetes between their teeth.

Harpoons or Spearguns would work.

Energy would quickly dissipate in water....

Ideas based on how aquatic animals defend themselves

Ink gun - allows the pc to become hidden and escape battle.

Slime gun - does damage by blocking the gills of a predator

Pufferfish Gun - acts like a shotgun that releases fake fish that are filled with toxins. When the Rad shark eats them they do damage .

Mirelurk Gauntlet - a crab crab claw with toxins

How about a one of http://scubabutch.com/wp-content/uploads/2011/03/UnderwaterScooter.jpg on the back of power armour where the jetpack would go. Maybe you'd have to find a suit of Navy (military not the colour) PA.

One issue with explosives when underwater is that the shockwave would rattle you too. A diver can get stunned or knocked out by the shock wave. Not sure power armor would be enough to prevent your head getting rattled by concussive force inside the helmet.

I would imagine experiencing a shock-wave in power armor would be little different than that of a submarine...it is going to hurt.

Anyways back on topic I just do not see how underwater combat could work given the design of the fallout 4. Like other said ballistic weapons (most likely including the Gauss rifle) would be extremely short ranged, explosives would not work....wait...I know!

You create a variant to the power armor turning it into basically a mini-sub and instead of a rocket launcher, its modded to launch torpedoes.

Ok that sounded a lot cooler in my head.....

What about that Unused Harpoon gun someone found?

Cryolator ! lol

I bet you could use it to make a nice shield? or an ice raft to float above the danger

Institute power armor and harpoon gun please!

Laser is made out of light even in the Fallout universe and light is able to travel at least some distance in the average Commonwealth water. Not that anyone really knows how a Fallout laser really works.

No underwater stuff please. Bioshock was dreary enough, don't want anymore of that nonsense.

Although someone on this forum mentioned like a sunken cruiser ship for Far Harbor, which would be pretty awesome area to explore, if it's Poseidon Adventure style.

laser does it's damage via heating the target. Air doesn't absorb that much energy, at least, not nearly on the scale that water does. I would rather have a projectile weapon (ballistic) than an energy weapon under water......