When shooting from a boat, it's especially important to pay close attention to the horizon line. It should be parallel with the top and bottom of the frame. I took this picture of Cape Lookout Lighthouse in the Outer Banks of North Carolina from a boat, and fortunately the water was calm so I could make the shoreline completely level.
If the waterline or horizon is slightly skewed, this can be fixed in Photoshop in post-processing. One way to do it (there are several techniques you can use) is to choose Select > all, and then Edit > transform > distort. A box forms around the picture with handles, and you can grab any of the corners to pull the image such that the crucial lines are parallel with the frame.
My settings for this picture were 1/500th of a second (I used a fast speed due to the movement of the boat), f/6.3, 200 ISO, and a 70-200mm lens set to 105mm focal length. When shooting from a boat, a tripod is not only irrelevant, it is a detriment.