...you need to not force the shutter speed. Try aperture priority at f16 and let the shutter speed fall where it needs to.
Yeah, Jarrod268 is right on. Your best bet is to use aperture-priority, which will automatically choose a shutter speed that is in the right ballpark.
Even aperture-priority may leave your exposure a bit "off" with particularly long shutter speeds though. So, probably the easiest way to ensure that you get the best exposure is to first take a shot in aperture-priority mode and then review the histogram. See if it is under- or over-exposed.
From there, you have two choices.
1) You can pop the camera into manual mode and just add or subtract a second or two of exposure as needed.
or
2) You can stay in aperture-priority mode and use exposure compensation to adjust shutter speed.
Either way will work fine. Manual mode will give you more direct and precise control over the exposure, while using compensation in aperture-priority will enable your camera to continue adjusting to slight changes in lighting while still producing roughly the same exposure that you dialed in after your test shot.
I even think that the pictures in the link above are made in daylight. What am I missing?
The photograph wasn't necessarily taken in broad daylight, though it may very well have been. However, even at dusk, you can
still achieve very bright photographs if your shutter speed is long enough. Honestly though, I wouldn't strive to "replicate" Hoya's sample photo... it is arguably over-exposed and exhibits "sun-burn" at the top of the waterfall where the highlights have completely blown out to pure white. This is generally considered undesirable and tends to occur when shooting waterfalls in poor, inconducive lighting conditions.