>In the past, I've been >using a polarizing filter on my lens
I thought that might be the case. CP filters tend to eat about 2 stops of light, which would be very helpful for what you are attempting.
>As for ISO, I've been using 100, so I'll try going lower.
Since you are shooting at mid-day, "lo" might be the best option here.
Aperture mode or manual exposure is probably your best bet since you will need to set the aperture to very small without the camera overriding it. After that, the camera will recommend at shutter speed, which will be close to what you need to get the exposure right. If that speed is too fast for what you are looking for artistically, then filters will be needed to block out more light.
If filters are not available, give this a try if your long exposure is to get the milky water effect: set the multiple exposure settings to "on" (page 141 in the PDF manual). The resulting image will contained blurred water since it changes in each exposure while the rest of the static image is sharp. While I have not tried this, I have read articles of it working quite well in a pinch.
---------+---------+---------+---------+---------+ Joseph K Seattle, WA, USA