I didn't say it was, only that it would do it, if the CoG is way beyond the aft limit, then the aircraft will be unstable around all 3 axis, not just pitch. Water is particularly bad because it is fluid and constantly moving the CoG for and aft left to right in addition to induced aerodynamic changes. A computer simulation will show you.
In light of DF's preceding post, if correct concludes my "any further" investigative thoughts