We do power off floatplane landings for training, though they are not always safely possible (so we choose the water/wind conditions for this training). Sometimes, landing with power is the only safe way to get a floatplane on the water. A power on landing affords a stretched out flare, so the flare altitude can be judged better, and adjusted (with power) if a mis-judgement is recognized. Power off, you get one chance. On a hard runway, a hard landing, or bounce is a nuisance, or possibly damaging. On water, a hard landing, or bounce, particularly power off, will much more likely result in the plane being upside down in the water. I have been right seat for this, it happens fast, and is very bad.
Waves pound the floatplane, and may result in the plane being thrown back into the air a little, perhaps slower than flying speed. The resulting second landing will be bad. Swells greater than a meter (for a Twin Otter) will certainly result in the airplane being thrown back into the air if you hit one wrong. I'm uncertain whether the subject Twin Otter ditched as a wheelplane or amphibian, I have heard it both ways. In the case of an amphibian, an open ocean water landing which resulted in the airplane remaining upright would be a happily very unusual event.
A floatplane Twin Otter in relatively calm (2 - 3 foot waves / no swells) ocean...
As for ferry tanks, if the ferry fuel could not be accessed in flight (as nearly happened to us once ferrying a Twin Otter) ditching a very heavy powerless Twin Otter in the open ocean would be an intense maneuver with little chance of a good outcome. Once in the water, full cabin tanks are hardly bouyant. An inverted floatplane with undamaged floats will float indefinitely. A Twin Otter wheelpane will sink when flooded. If the pilots opened the cockpit doors before the ditching, flooding the cabin could happen fairly quickly. A crash into the water can result in unexpectedly high crash forces (we exceeded 14G). And underwater egress is a skill (I retrained for it two weeks ago). So, if things did not go well, fatalities in the pilot's seats would be possible.