I'm on version 0.15.3
The mission to photograph 18 seahorses can not be completed because two of the seahorses can not be photographed. There is no problem with the other 16.
Here's a photo of the problem. I'm at the central mast. Various angles and distances were tried without success in triggering the green boxes.
Does the red box appear?
No. There is no box appearing for me. This problem still exist in the current release 0.15.6
You can click the photo above for a better view of the situation. I've tried every angle, but no luck.
The bug that seahorses are stuck and unable to move will be fixed in the next update. This should allow the mission to be completed properly.
I have report this Problems already also. :)