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An NPC is casting Heat Metal on a longsword wielded by a PC inside a Wall of Water. The description for the Wall of Water spell states (emphasis mine):

Any ranged weapon attack that enters the wall’s space has disadvantage on the attack roll, and fire damage is halved if the fire effect passes through the wall to reach its target.

Meanwhile, Heat Metal indicates the fire damage comes from touching the weapon, which is inside the wall:

Any creature in physical contact with the object takes 2d8 fire damage when you cast the spell.

The words "fire effect passes through" seem to be ambiguous here, as the spell caused the effect, but the physical contact caused the damage. Am I correct in assuming the victim will take the full 2d8 fire damage here?

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Heat metal deals full damage to the creature

fire damage is halved if the fire effect passes through the wall to reach its target

The fire effect is not passing through the wall to reach its target. The creature is touching the heated metal object. Heat metal says

Choose a manufactured metal object, such as a metal weapon or a suit of heavy or medium metal armor, that you can see within range. You cause the object to glow red-hot.

While spell targets the metal object through the wall, it is not causing the fire effect to travel through the wall to reach it. Instead, the object itself glows red hot.

Note however that there is another rule, that then reduces the damage to the creature by providing resistance to a creature that is fully immersed in water, as is the case here (Underwater Combat, p. 198, PHB, thank you to @PeterCordes for pointing this out):

Creatures and objects that are fully immersed in water have resistance to fire damage.

So heat metal will deal full damage, which then is halved by Resistance.

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    \$\begingroup\$ On the plus side, the PC can place their hands under running water at least (: \$\endgroup\$
    – Robotnik
    Commented Jun 24 at 0:43
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    \$\begingroup\$ In fact they're resistant to the initial damage even if they weren't already: Creatures and objects that are fully immersed in water have resistance to fire damage. - 5thsrd.org/combat/underwater_combat . The special rule in the spell description for Wall of Water indeed doesn't apply, but if this water is real water and someone's fully surrounded by water, that would count as "fully immersed" so the general-case rule applies. \$\endgroup\$ Commented Jun 24 at 6:42
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    \$\begingroup\$ @PeterCordes A wall of water is 1 foot thick, so I don't think that necessarily applies to "fully immersed" although I'll admit if you're holding your longsword inside the wall when it gets heated, it would apply... \$\endgroup\$ Commented Jun 24 at 20:41
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You could potentially rule either way on the Wall of Water spell doing something special to fire magic that pass through part of the wall, but I think the intent is for that halving to apply to spells like Fireball and Fire Bolt where something hot actually travels from the caster to the target. Not just the "line of effect" from the caster to the metal object; heat doesn't travel across that path, just magic which creates heat when it gets there.

But depending on how the DM wants magic to work in their campaign, the magic of Wall of Water could have a special interaction with the magic of any fire spells, making its special abjuration properties more potent.

Note the wording says "the fire effect". For Fireball, the "bright streak" that flies from the caster's finger to the target point is part of the effect.
For Heat Metal, the actual effect only happens on the target object; there's no description of anything happening between the caster and the target.

So this strongly favours the interpretation where the Wall of Water special halving doesn't apply, so you'd only have resistance from being in water, not also halving it separately from that.

Creatures and objects that are fully immersed in water have resistance to fire damage.

https://5thsrd.org/combat/underwater_combat

If the Wall of Water is real water and someone's fully surrounded by it, that would count as "fully immersed" so the general-case rule applies. But note that a Wall of Water is only 1 foot thick, so Medium creatures might have a hard time being fully immersed. If the hand holding the metal object is inside the water, it could apply then.

Note that casting a Fireball or Fire Bolt through a mundane waterfall at a target standing in air on the other side does not halve the damage. Wall of Water is special that way. The resistance effect from mundane water only applies to targets actually in water, not spells passing through.

The mundane-water effect is resistance, so it doesn't help if you're already resistant, unlike Wall of Water which halves the damage separately.

If someone casts Fire Bolt at you while you're standing in Wall of Water (or taking a mundane bath with a Wall of Water interposed between you and the caster), you would take 1/4 damage. Halved by the Wall of Water special effect, halved again by resistance. If it was a Fireball and you made your save, halved yet again for 1/8th damage.

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