Car alternators are made to spin at higher revolutions than hydro and wind power typically generate. You can change the gear/pulley ratio, but this compounded by the need to generate an electric field in order to create
energy and starts to become problematic and fairly inefficient. Permanent Magnet Alternators/Generators (PMA's, PMG's) do away with the need to create a magnetic field electrically.
Missouri Wind and Solar sells a bunch of these for both wind and hydro power generators. They have a bunch of videos on YouTube that are worth a look.
There are also other places
online that
sell both custom PMA's as well as ones made from old car alternators. You can even buy kits to rebuild an alternator you may have on hand to save money. I haven't looked at it in a while, but MWandS had a bunch of them still for sale on ebay. They usually have lots of things on their ebay store on sale if you are willing to wait and keep an eye out there. There are other brands out there, but some of them take significant shortcuts to shave a few dollars, so it is worth investigating before throwing down any cash.
Also, the amount of power you can create will be determined by the amount of flow and head your
water source has. A PMA would need a sizable water wheel to create power and an alternator would need an even larger wheel to produce the same amount of excess energy.