Regarding the ferry fuel being there, though not being available for use: I don't think so,
This topic is probably at the point where some more facts are needed, and further thinking then based upon those facts. It is possible for an airplane, including a ferry configured Twin Otter, to have fuel aboard which is not available for use in flight - it happened to me this year in another type, and happened while I was right seat ferrying a Twin Otter in the mid '80's.
As to wheelplanes floating after ditching, many float for a long time, while wings slowly fill with water. Is there any factual information as to how long this Twin Otter floated?
Whether ferry fuel flow was a factor in this sad event is up for more factual information. In the mean time, the accident is a reminder about the importance of understanding the fuel system of your airplane as you're flying it, and knowing when to change plans - That is fresh in my mind!