Though there are a few sites in England where otters are regularly seen, they are generally much easier to find in Scotland. The Shetland Islands probably offer the best chance, closely rivalled by Skye and Mull.
A few years ago I spent two days otter watching on the Isle of Mull and saw six different otters, but I was making a real effort.
The secret of success is to drive along the coastal roads, stopping frequently to look for them. They can often be seen sitting on rocks, or fishing, when the so-called ‘ring of bright water’ often marks their presence.
The otters on Skye and Mull clearly feel safe from persecution, so are active during the day and seem to take little notice of people watching them at a distance. You will need patience and good binoculars to be sure of success