I had a feeling, so I asked Google:
Based on the classic British idiom, it is highly likely that Stranraer is, indeed, wetter than an otter’s pocket.
"Wetter than an otter's pocket" is a phrase used to describe intense, soaking rain. Stranraer, located in South West Scotland, is frequently associated with heavy rain, with reports noting high rainfall, such as 20mm in under six hours.
The phrase was notably used in the Daily Record to describe torrential rain in the area.
It is often used in the UK to signify a state of being completely drenched.
An "otter's pocket" is often imagined as the ultimate, albeit mythical, state of wetness (though sometimes jokingly referred to as a skin fold under the armpit used for storing pebbles).
Therefore, in the context of weather, Stranraer's notoriously wet climate often fulfills this description.