Question: In July 2019, the [United Kingdom seized an oil tanker carrying oil from Iran to Syria](https://www.theguardian.com/world/2019/jul/04/royal-marines-gibraltar-tanker-oil-syria-eu-sanctions). Reportedly, this is to enforce EU sanctions against Syria. But neither Iran nor Syria are members of the EU, so how can EU sanctions against Syria be of any relevance? Do sanctions imply a [blockade](https://en.wikipedia.org/wiki/Blockade)? Do EU sanctions authorise EU member states to check and possibly impede any vessel transiting through EU waters, or is such passage supposed to be free under [innocent passage](https://en.wikipedia.org/wiki/Innocent_passage), [strait passage](https://en.wikipedia.org/wiki/Strait_passage), or [transit passage](https://en.wikipedia.org/wiki/Transit_passage)?

(It gets more complicated, because it was in waters claimed by Gibraltar, a claim that Spain disputes, and the ship was apparently registered in Panama; but I'm not sure those complications affect the answer in this case)

Answer: 
The key legal point of their justification seems to be this, from the [press release](https://www.gibraltar.gov.gi/press-releases/grace-1-confirmed-to-be-carrying-full-load-of-crude-oil-5222019-5112) linked and quoted by Fizz:

> 
> The Grace 1 was detained last week in Gibraltar when it freely
>  navigated into British Gibraltar Territorial Waters to a point two
>  miles off the Eastside of Gibraltar, having previously exited the
>  international waters of the Straits of Gibraltar, on a pre-arranged
>  call for provisions and spare parts.
> 
> 
> 

Article 18 of [UNCLOS](https://www.un.org/depts/los/convention_agreements/texts/unclos/unclos_e.pdf) says this:

> 
> 1. Passage means navigation through the territorial sea for the purpose of:
> 
> 
> (a) traversing that sea without entering internal waters
>  or calling at a roadstead or port facility outside internal waters; or
> 
> 
> (b) proceeding to or from internal waters or a call at such roadstead
>  or port facility.
> 2. Passage shall be continuous and expeditious. However, passage includes stopping and anchoring, but only in so far as the same are
>  incidental to ordinary navigation or are rendered necessary by
>  force majeure or distress or for the purpose of rendering assistance
>  to persons, ships or aircraft in danger or distress.
> 
> 
> 

Once you stop to take on supplies, it's no longer passage.

I also don't think stopping to take on supplies in itself counts as calling at a [roadstead](https://en.wikipedia.org/wiki/Roadstead) or port facility. But even if it did, "passage" only seems to apply to going to or from the place, not actually being at it. Once you've chosen to stop in a country's internal waters or facility, you're placing yourself fully within its jurisdiction.
I intended to post a comment, but I require 50 reputation for that, so will post my comment here. Apologies for this.

[This article](https://7dnews.com/news/iran-s-grace-1-captain-asks-to-resign) reports:

> 
> Grace 1 was supposed to undergo maintenance before being captured, but
>  the latest events disrupted installation of spare parts, which
>  rendered the tanker incapable of undertaking a long voyage.
> 
> 
> 

If Grace 1 was unable to continue its journey without maintenance, it could be argued that this constitutes an emergency as set out in international shipping laws, and thus its stop in Gibraltar for maintenance falls within innocent passage.