It's a common mistake to use shutter priority because you want a slower shutter speed: you're thinking mainly about exposure time so having full control over the shutter speed seems the right way to go.
Unfortunately, the problem is that cameras have a vast range of shutter speeds (mine goes from 30 seconds to 1/8000th of a second) but a comparatively tiny range of apertures (the lens that's on my camera at the moment goes from f/4 to f/22). f/22 is only around 30 times faster than f/4, but 1/8000s is around 240,000 times faster than 30s! Even if we limit ourselves to shutter speeds that are good enough to shoot hand-held (say, 1/30s and faster), my fastest shutter speed is still around 260 times faster than the slowest.
Or put simply: let your camera choose the shutter speed and it's got lots of latitude to play with. Let it choose the aperture and it hasn't. This is why using shutter priority so often ends up with a shot that's badly over- or under-exposed: the camera simply ran out of options.
(I'm ignoring ISO for now to keep things simple but on a typical camera ISO has a similarly small range to aperture. For example, ISO 1600 is only 16 times faster than ISO 100. And if you prefer a lower ISO setting to keep noise down, there's even less room for manoeuvre there.)
So, a much better place to start with your fountain shots is to let the camera choose the slowest shutter speed it can manage:
- Set the camera to aperture priority and choose your smallest aperture
- Set the ISO setting as low as it will go
- If you have one, use a neutral density filter to darken the scene even further
Now take a shot and see how the shutter speed works out. If it's too slow, just ease back on the aperture and other settings until you get the shutter speed you want.