Gibraltar has been ruled by Britain since 1713.
Royal Navy’s Gibraltar Squadron keeps watch over the British territory's coastline, as well as escorting visiting ships and submarines.
The squadron's role has recently become more important than ever, with Brexit reigniting Spain's claim on the territory.
It patrols the sea every day, covering an area from the shoreline to around three miles out, as well as escorting and protecting other Royal Navy vessels.
"What we’re out there really to do is to try and encourage good behaviour," said Commanding Officer of Gibraltar Squadron, Lieutenant Commander Kyle Walkley, outlining their role to challenge vessels not acting "in accordance with international law".
"In the instance of any bad behaviour, that will be challenged, and if necessary, we will ask those vessels to leave."
The Navy operates two patrol boats in the area, HMS Scimitar and HMS Sabre, as well as a number of Rigid Inflatable Boats (RIBs).
"They're generally quite reliable," said Petty Officer Chris Ramsay.
"I think that's probably the best things about some the old tech - if it goes wrong, it's quite easy to fix. Generally they're quite nimble, fairly fast off the line. They're not bad little boats."
Deputy Chief Minister of Gibraltar’s government, Dr Joseph Garcia told Forces News the vessels can be very busy, especially when legal boundaries are ignored.
"The reality is, because they take this view - which has no basis in law - that the waters are Spanish, they tend to behave in the waters as if they belong to them."
Dr Garcia said the UK and Gibraltar "challenge each and every one of those incursions, both physically on the water and also with diplomatic notes and protests afterwards".
"The number [of incursions] goes up and down, there could be months when there are 50, less than 50 or more," he said, stressing the importance of dealing with each incursion "carefully".
"We are literally attached to Spain - we share a land border with Spain here," Commander British Forces Gibraltar, Commodore Tim Henry said.
"The waters around Gibraltar will clearly be transited - Spanish vessels will go through those waters.
"The majority of the time, that's perfectly acceptable and normal in international law. And don't forget that we do the same, the Royal Navy does the same, elsewhere in the world," he added.
"Occasionally, a Spanish state vessel, or another nation's vessel, may act inappropriately inside of British Gibraltar territorial waters - may not act in accordance with international law."
Cover image: Patrolling the Gibraltar coastline in a rib.