Long lens photography is not easy. It takes a good level of knowledge, skill, and lots of practise. Even then, expect plenty of disappointments.
There are many possible reasons for lack of sharpness. Assuming your camera and lens* are performing as they should and images are properly post-processed, reasons include: inaccurate focus, slow shutter speed, dull overcast light, high ISO, under-exposure, excessive cropping (enlarging a small area of the image), and atmospheric pollution/moisture that is more common over water. Your shutter speed should be sufficient, but apart from that I suspect you may be suffering all the other problems.
*check your lens is properly calibrated and focusing accurately.