A group of 4 adventurers has Water Walk cast upon them and are walking across a river towards a pirate ship. The dread pirate wizard sees them and casts dispel magic.

Looking at Dispel Magic, it says you can "Choose one creature, object, or magical effect within range."

Can the dread pirate wizard select the Water Walk effect and dispel the Water Walking on all 4 adventurers at once (presuming they are all within 120 feet)?

Or can he only dispel the effect on a single adventurer?

Dispel magic would end the effect on a single adventurer. Here is the SAC entry that causes me to say so:

If dispel magic targets the magical effect from bless cast by a cleric, does it remove the effect on all the targets?

Dispel magic ends a spell on one target. It doesn’t end the same spell on other targets.

When Dispel Magic references a "magical effect", it is talking about something affecting an area, rather than specific creatures or objects.

For instance, there are spells that make an area become difficult terrain; the ground affected isn't a creature, nor is it an object; in such a case, you would Dispel the effect on the area of ground.

I second (third?) that it only dispels the effect on 1 target creature.

I guess I wasn't clear.

As the 'magical effect' is strictly an area, the only option in the presented scenario is indeed to target 1 creature only.