A coherent approach to using English or Scottish naval ships for Scotland's naval needs had been lacking over the first three decades of regal union. However, despite such inconsistencies, an ideological underpinning for one navy controlling the seas around the British Isles was being constructed. The formative years of the regal union coincided with a debate over whether the seas were a free commons or the property of imperial monarchies. Dutchman Hugo Grotius published his work Mare Liberum in 1609, attacking the Spanish and Portuguese claims to exclusive rights in the Indies. Serafim de Freitas, a Portuguese friar, published a riposte in 1625. The Stuarts muscled in on this dispute between partisans of the Dutch and Habsburgs, expanding upon the historic Scottish rights to the assize herring and encouraging the production of tracts to support their dominion over British seas from, amongst others, John Selden, whose text became the cornerstone of an increasingly energetic English assertion of maritime sovereignty over the next century.

Mare Clausum

The assertion that the seas around Britain were part of the Stuart patrimony was partially a Scottish addition to naval practice in the British kingdoms. Elizabethan England had held to the freedom of the seas, which justified incursions into the Spanish-held Indies. The Scots, on the other hand, had claimed exclusive rights over their fishing grounds since the thirteenth century, exercised, in theory, with the exaction of the assize herring levied on everyone fishing in Scottish waters. This would be developed to underpin the claim to sovereignty of the seas that Charles would make, which can therefore be described as ‘Scoto-British’. De Domino Maris, the most important work of William Welwood, a professor at St Andrews university, argued that oceans were Mare Liberum, but waters near to land could be subject to sovereignty. De Domino Maris was a response to Mare Liberum of 1609, to which Selden's Mare Clausum, published in 1635, was a far more famous riposte.

Selden had initially submitted his work to James, but it languished until Charles asked Selden for a rewrite. The arguments against the Grotian position do not concern this study, but how the composite monarchy was broached does. Selden's work acknowledged the importance of Scottish claims, saying that the British seas were ‘possest by the English, Scots, and Irish’, and Scottish waters had a separate chapter in his work.