For my experience ferrying Twin Otters with cabin fuel, it is vital to assure that the cabin fuel venting is working as required while still within return to safe landing range on the belly tanks. On one of our flights, my captain, very aware of this, spent 45 minutes in the cabin sorting out a venting problem so we could continue, reminding me that if he could not get the cabin fuel venting, we were headed for an overweight landing (that leg was not over water). He got it venting, and our further legs were trouble free with caution. Careless filling of the tank can splash fuel out the vent, and the "U tube" of fuel in the vent line prevents correct venting of the tanks. I know of a Twin Otter which ditched many years ago because the pilots flew beyond the return point, and could not use cabin fuel.
An overweight ditching (even on floats) in open ocean would be very worrisome!
I have no special knowledge of this event, but reported details thus far bring to mind experiences of the past....