I added the waterjug, also trying with the smallwaterbottle, to the kit on my server (plugin rust kits by wisphers88), having them in the inventory full of water, when I created the kit;
But when I go to get the kit I give the waterjug and smallwaterbottle completely empty, someone know how to fix this, thanks.

not possible unfortunately. You could add pumpkins and corn as they will hydrate players

I think cactus flesh gives most hydration