Water based ink is a "thin" water consistent liquid. This ink is used for any "rotating" self-inked stamp. Dater stamps are one example of a self-inked stamp. Our Shiny brand stamp on the website here is also an example of a self-inking stamp.
If you have any questions and are not comfortable choosing an ink for your stamp feel free to email or call us and we can decipher what type of stamp you have and make sure we get you the proper ink.
Notice: Using the wrong type of ink can ruin your stamp.