All rollerball refills are water-based ink. The Waterman rollerball refills have a fine point and are available in black or blue ink.
Your Waterman rollerball pen will also fit a Waterman ballpoint refill. Those are available in black or blue oil-based, waxy ink, with a fine or medium point.
Depending on the age and model, your Waterman rollerball may also fit any brand of standard-sized rollerball refill. If you want something other than blue or black ink, you may wish to try a Monteverde branded refill like the popular R4688.