Too much light. Have you read that doc I sent you? It tells you what affects the light sensitivity.
Make sure you ISO is at it's lowest ... if needed then certainly take the F-stop way up as you did. If you still can't get the shot then you are going to need to either wait for lower light or artifically create that effect with a filter attached to the lens. That's what Neutral Density (ND) filters do. You buy them in different darkness's which then allows you to lower that shutter speed further.
I use a circular polarizer which kinda does the same thing with the added advantage of being good for cutting reflections as well. You get two uses out of that way. ND or ND Graduated filters are the right tool for that job though. Usually. If there's a ton of reflection off the water then the CP can be the better choice.
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