Familiars are spirits, they can be summoned. But there isn't really any 'Water' ones, it's all based on how they appear to you, generally as an animal but not always. Also you don't need a 'water based animal'.
Familiars are spirits, not physical beings, the idea that they were physical came during the 'Witch Hysteria' where people preyed on other people sometimes calling their pet, or an animal that spent a lot of time at the persons residence their Familiar. To use this as fuel to harm them, generally it was Elderly widowed or spinisters who got these charges.
Finding a familiar can be done several ways, you can call up a spirit and form a working relationship with them to the point where they agree to be your familiar, a spirit can also approach you as well as they can be 'made' like servitors, thoughtforms etc