Hi there
Just saw you're heading to Mull soon. I'm working here (lucky me!) and have seen otters several times in Calgary Bay. Loch na Keal bay is supposed to be really good too. Best when tide is low and when starting to come in. I worked on Skokholm once too so got plenty of puffins. It's too early in the season for them still. They arrive late April/early May but no sand eels in bill til late June / July when they start feeding young.
You'll just have to keep coming back to these amazing places lots of times! Hope some of this is helpful.