Cymbel said:

With the Shock upgrade. Walking along with the whine of servos and pneumatic pistons, and sparks fizzling off of discharge nodes.

Kasatka said:

Reminds me of a Storm Knight from Iron Kingdoms/Warmachine.

People in Power Armour can't swim.

Which means you should get the party onto a boat. Then sink it.

Watch them squirm as they realize all their big, expensive, rare guns and toys will drag them to their watery grave unless they let them go.

And let the PA wearing fella try to figure out how to remove his armour, on his own, real-quick, while holding his breath and sinking like a rock.

Power armour is normally fully sealed with oxygen supply and rebreather.

You will need to make it very deep water or the PC will just walk to the shore. Not that I think such engineered situations make for a good play experience…

Fresnel said:

Indeed, Power armour is by default environmentally sealed with an oxygen supply that'll often outlast it's power supply (unless we're talking the infinite duration fusion stacks that are the Astartes grade power sources). Sure they can't swim but they will have senses that can pierce the murkiness of water and unless they go REALLY deep, like mariana trench deep, their power armour will survive the pressures.

That said, water can be a really good impediment to power armour characters - the pre-written adventure in the Deathwatch book First Founding (iirc) is set on an oceanic world with floating hive cities. The cities get attacked by hostile forces and the PCs potentially have to fight their way out of a capsizing metal behemoth of a hive city, flooding, sloping and full of hostiles. Environmental challenges can be as tough as combat challenges…

Kasatka said:

This is the Grim Dark of the 41st Millenium of course they are going REALLY deep. 

There aren't any ponds in the Imperium.