You should probably get a vehicle as a full keg was pretty heavy last time I had one.
If it’s an item, just pick it up and examine a water tile (pool, river, etc), then select the keg as the target container. From there, you can pull water from the keg to use in boiling water, same as you would any other recipe.
If you’re talking about the non-portable wooden kegs, you’re gonna need to manually bring water to it, fill it, and manually draw from it. So I’d fill those with clean water. Sorry.