Yell's otters live along the voes — the long, narrow sea inlets that cut deep into the island's coast from the east and west — and because they hunt in daylight along an open shoreline rather than in rivers at night, they are genuinely watchable in a way that mainland British otters rarely are. The best approach is to park somewhere with a view of a voe, sit quietly above the waterline, and wait. Dawn and dusk are the highest-probability windows, but on a calm summer day otters can appear at almost any hour. The main road along the island's spine passes within sight of several promising stretches of water.
When you find one, watch what it does rather than trying to get closer. A Yell otter working a shoreline is methodical: diving repeatedly in the same patch, surfacing to eat on a favoured rock, shaking the water from its coat, diving again. They are not tame and will vanish quickly if you move too fast or too close, but a careful observer at fifty metres will often be ignored entirely. The moment of cracking a crab shell, or hauling a butterfish onto a weed-covered rock while a grey sea moves quietly below — this is what Yell otters are for, and it is considerable.