Grabbing an Aqua Sac in Monster Hunter Wilds is important early on if you want to craft a water element weapon from the Balahara tree. Since you'll find yourself in the fiery Oilwell Basin before too long, there's no harm in acquiring a weapon that deals water damage for a little extra effectiveness against those fire monsters.
Like other sac monster materials, there's only one main way to get an Aqua Sac so you can craft that Balahara glaive, and unsurprisingly, it involves fighting a Balahara. All that said, here's how to get an Aqua Sac in Monster Hunter Wilds.
Monster Hunter Wilds Aqua Sac location
You can get an Aqua Sac as a target reward for hunting large water monsters—target reward means one you get randomly at the end of a hunt vs. those you carve or acquire by destroying wounds. The earliest water monster you fight is a Balahara, so this is your best bet. But wait, isn't the Balahara some kind of sand worm? How is that a water monster?
This may be true, but the Balahara applies waterblight when it spits from its mouth, so despite its rough sandy exterior, it has a soft watery core. What am I a biologist? If you look in the Monster Field Guide, you'll see that there's an 18% chance to get an Aqua Sac as a target reward from the Balahara. After Balahara, your next best bet is Uth Duna, but for that monster it's only a 16% chance as a target reward.
Essentially, you only ever get elemental sacs as target rewards from a hunt, so it's mainly just down to chance whether one drops. Later on, once in High Rank, you'll unlock the Material Gathering mechanic in the Forbidden Lands' villages, and these can potentially get you sacs also. The Supply Ship that also unlocks at the same time will sometimes sell sacs for Guild Points too.
