Howdy Molly, to accomplish a panorama that doesn't have this bend to it, you need to rotate on the
nodal point for your camera and lens combination, this can be accomplished using a panoramic head like the
Nodal Ninja, although there are many cheaper models out there that operate the same. You would also need to have taken several rows of images, especially around the centre section of the image, so you have all that room to get it stitched together again, but Photoshop doesn't cope very well with this sort of task trying to stitch together multiple rows of panoramas, you need to use something like
Autopano which makes light work of it.
Ideally, you would have been a bit further away from the shoreline too, this helps to lessen the curvature effect. If you are using Photoshop you could try choosing 'Rectilinear' as the output type for your panorama and see if that makes a difference to the curvature seen above, it should, but my suspicion is that it will squish the centre of the image dramatically from the top to the bottom, as you may not have taken multiple rows of images to compensate for using this assembly method.