Many Variables. It matters if this was a recent tatoo, or an old one. It probably was not water but a serous fluid (the watery part of blood). Or maybe your body was reacting to some of the pigment, and the body put the fluid around the pigment to isolate it. Typically a pimple is red, raised and has a central white core, and the fluid inside is pus not like water.
Infection. It could just be a pimple . . . Or it could be an infection by the bacterium staphylococcus aureus. Because this is on, or near, your new tattoo, you should make an appointment to see your physician now. Because of the likelihood that this is an infection, you don't want to wait around for it to possibly get worse. Treatment would include a course of topical or oral antibiotics.
See a Dermatologist. What you describe is a vesicle and together with the itch may indicate that you may have an allergic reaction to one of the colors in your tattoo. It is also possible that the vesicle is unrelated to the tattoo and is from something your skin is allergic to and that area is coming into contact with. Shingles is another possibility but there would be a cluster of vesicles, pain & burning.