As the others have said you need to be lucky, the more time you spend by the water the luckier you will get!
When I used to fish the East Devon rivers (Axe and Otter, although I never saw one on the river Otter
) regularly I would see otters up to half a dozen times a year.
Nearly all my sightings were in the last hour before dark, I expect dawn would be as good but I never was very good at getting up in the mornings! They seem to become fairly bold in the evening and at night, every time I saw them they passed within a few yards of me and often stopped for a look. There was one place a friend used to fish where he couldn't leave a fish lying about on the bank at night without it being stolen.
The ones I have seen in full daylight have usually spotted me quickly and then they just vanish without trace.
I think all you can do is find an area you know they are in and sit and wait. Hope you see one soon.
Andy