The camera can be set to lock the exposure in auto modes using different buttons, but a system default is to lock in the exposure at the time you hold the shutter release half way down. So, you'd get to where you want to take a shot, go halfway down, the autofocus and exposure would lock in, and then you'd take the shot.

Since you were using partial metering in a scene that was dramatically different between shaded area and unshaded, my guess is that your bobbing up and down in a kayak caused you to meter mostly in the shade for one frame and mostly in the sun for the other.

You probably locked the exposure while the meter was located here on this frame:

and probably locked in while metering here on the other:

Subtle differences for sure, but enough to cause a big exposure calculation change.

For a scene like this, it's most important not to blow the highlights. If you shot RAW, it actually appears that your brighter shot may be the better of the two in the end, as you can tone down the exposure in post (assuming the highlights are salvageable).

If you only get one shot at the image (like kayaking down a river), then I'd tend to use evaluative or center weighted average, making sure to meter off averages in the scene, not the brightest or darkest points.

Also, there are custom functions that allow you to decouple the exposure lock and autofocus from the shutter release. I'm personally a fan of back-button focus and the shutter release used as exposure lock, but there are other options. I'd advise doing this so that you can figure out where to point your camera to get a good meter reading for the scene using a mode you are comfortable with, lock that in, and then recompose and focus and shoot.