Water Runecrafting Gloves are supposed to provide x2 exp for Water runecrafting. They're not expected to provide unlimited water runes.
"Water runecrafting gloves are a reward that may be purchased with tokens from the Fist of Guthix minigame or from Stanley Limelight for 80 thaler. They give double runecrafting experience when crafting water runes. This effect lasts for the crafting of 1,000 rune essence, with a total of 12,000 experience (Net gain 6,000 experience) after which they crumble to dust."