I've been to Mull a few times .... what a wonderful island!
I have also seen Otters there on several occasions but sorry I can not remember the precise locations. I believe Mull has a good population of Otters, so basically try any stretch of coastline with rocks, seaweed, etc. On Mull and elsewhere in NW Scotland, I have tended to be more successful at the beginning or the end of the day and on a rising tide. Good luck!
I am in NW Scotand again in June of this year and will be taking a day trip over to Mull to see the White-tailed Eagles.
A couple of useful links ....
Mull Birds and
RSPB viewing hide
Richard