These allegations do not stand up to scrutiny:
The Scottish Environmental Protection Agency (SEPA), whose role is to ensure the environment and human health are protected, set medicinal quantities for use based on an individual farm site's environmental capacity to assimilate the use of medicines within the environmental quality standards (EQS). The EQS are highly precautionary levels set to protect the most sensitive of species, in the case of Scottish salmon farms that is crustaceans.
Based on the levels of medicines discharged into the marine environment from a farm treatment, a human weighing 75kg (11.8 stone) would need to ingest in excess of 300 million litres of seawater (based on 250ng/kg 3 hour EQS) in order for levels to be of concern. For context a person living to 80 years of age will ingest 87,600 litres of fresh water, based on consuming the recommended daily amount of 2.5 to 3 litres per day.
The article cited uncorroborated research purporting to show that currents could cause organophosphates "to gather in significant concentrations, above 40ng/l, which is is the environmental standard maximum limit set by Sepa." On the basis of the article this research does not appear to have considered that azamethiphos is soluble in water, meaning that it degrades in a matter of days (<5.6) after release and in doing so produces non-toxic transformation products.
Ignoring the fact that azamethiphos quickly breaks down into non-toxic substances, a swimmer would need to ingest in excess of 1.8 billion litres of water based on the 40 ng/l (72 hour EQS) to reach the acute toxic level.
It is also worth noting that organophosphates are widely present in the environment with multiple uses including within fruit and vegetable production, and are the main component of Sheep Dip. They are also a major component in non-prescription flea collars widely used by UK pet owners.
The Scottish Salmon Producers Organisation (SSPO) and water safety organisations advise that all recreational marine users stay outside the buoys that mark the boundaries of farms because of the unseen physical hazards of nets, mooring, pipes and anchors.