So, as the ship founders, the canniest crew members push old Flabbo overboard and proceed to climb aboard.
The crew, anticipating just such an event, had fed him only pork fat for months, increasing buoyancy!
Then, there's only to wait for a friendly crew of Pirates, Polynesians, or the odd white whale to pick them up and shunt them to the nearest port tavern.
Personally, I think any hungry sharks, barracudas or orcas, if they catch sight of him will be eating their way through him to the crew. Still poorly thought out.