Underwater Sigils?
The cheapest ones /sarcasm
It really depends what you want out of your weapon, they work just like above ground sigils so basically copying one from there onto it would probably be best (most likely a “On kill stacking” sigil so going underwater doesnt slow your stack gainining)
But really… personally I wouldn’t bother Im just using what ever was on the weapon when it dropped myself (heck my harpoon gun is still a level 50 green because I use it that little)
Given how limited our stealth options are underwater, and therefore how lacking our condition removal is, I go with a sigil that removes conditions on a 10s cooldown on at least one of my weapons.
Stacks drop when you exit the water though.
They don’t (for me at least).
I personally wouldn’t waste a “on kill” sigil underwater. 80% of the game your above water, so it’s better to bring dps/utility instead. And if you know you’ll be underwater for a while, just prep on trash (yellow mobs also count) first. Really, suggestions are heal on crit, bleed on crit, sigil of generosity, or any DPS (fire, lightning, force). I tend to use Bleed on Crit on my harpoon gun to have it double-bleed stack and DPS on the spear.