Ferries on the Irish Sea are among the targets of an international campaign to protect crew members from exploitation.
The International Transport Workers Federation (ITF) and its affiliated unions will send investigators aboard vessels at ports around northern Europe in the coming days.
The inspections will target vessels flying a national flag and flag-of-convenience vessels, which are usually registered in countries with lax regulations.
The operation will include checking conditions, recovering owed wages, and seeking to establish agreements to protect workers.
ITF affiliate in Ireland, Siptu, found that out of 82 inspections in the 13 months, wage arrears worth €700,000 had been recovered from 21 vessels and more was still outstanding.
Serious health and safety concerns compounded by the increased use of untrained labour were also highlighted today as he ITF launched its week of action in Ireland today.
Siptu general president Jack O'Connor said lack of "enforcement and the political will to act in defence of workers' rights" was largely responsible for low standards on the Irish Sea.
"This is all the more deplorable, and short sighted, given that we operate within an economy where 90 per cent of our imports and exports are transported by sea," Mr O'Connor added.
The issue came to the fore in late 2005 when a bitter dispute broke at Irish Ferries over it plan to make over 500 crew members redundant and replace them with foreign workers on lower pay, terms and conditions.
Participating countries in the week of action are: Belgium, Denmark, Estonia, Finland, France, Germany, Ireland, Latvia, Lithuania, The Netherlands, Norway, Poland, Russia, Sweden, the UK, and Norway.