The Swellies, Mr Jones explains, are particularly nasty tidal currents in the strait that runs between the Welsh mainland and Ynys Mon, the island of Anglesey.
Misjudging the Swellies can land you on the Swelly, one of two dangerous rocks in Mr Jones's stretch of the strait. The other is the Platters. Over the years, many an ignorant or complacent skipper has found his boat turning round in circles while being borne at great speed towards one rock or the other.
"The Caernarfon Harbour Trust area is quite challenging, for apart from fast tidal streams like the Swellies, there's also a steep tidal range of between six and eight metres," says Mr Jones.
The harbour trust was established in 1793, when Caernarfon was exporting Welsh slate and was a busy commercial port. Now it is given over to leisure boats. "On the whole, skippers are very good, but you still get the odd one who isn't aware of the dangers of these beautiful waters, and who will go out without charts or flares."
Mr Jones became harbour master at Caernarfon on the retirement of Captain GW Jones. At 31, Mr Jones (no relation) was one of the youngest candidates in a crowded race for the job. In his favour was the fact that the Menai Strait is in his blood. For seven generations members of his family have piloted boats successfully between Puffin Island and Caernarfon.
But he had more than his experience as a pilot to offer. "My FE track record weighed heavily in my favour," he says. Mr Jones has successfully completed a diploma in port management, through Lloyd's List and Informa Distance Learning, paid for by the harbour trust.
FE will play an increasingly important part in the careers of a new wave of harbour masters, more and more of whom are "misters" rather than "captains".
Mr Jones says: "Traditionally, harbour masters were former sea captains, but there aren't as many of them nowadays because so many shipping companies go foreign flag."
The job of harbour master is also changing, he adds, with greater demands for compliance and accountability, as well as for the ability to cope with the explosion in leisure sailing.
The port industry established Port Skills and Safety Ltd at the beginning of 2002 to set and maintain health, safety, skills and standards. PSSL's subscriber base now exceeds 200 ports and port-related organisations, employing more than 19,500 of the sector's estimated workforce of 25,500.
The ports sector operates level 2 qualifications for stevedoring, passenger and marine operations, accounting for nearly two-thirds of the sector's employees.
Funded by the Government, PSSL is producing new standards in FE, based on international best practice, due for publication next spring. The Learning and Skills Council is also a partner on a pilot project to increase uptake of vocational qualifications through approved training modules .
PSSL has also completed the harbour management national vocational qualifications which underpin the Port Marine Safety Code.
On the other side of the harbour wall, the shipping and fishing industries have formed a Maritime Skills Alliance to plan for the future.
The port industry may be changing, but the Swellies swirl on. The key to a safe passage, says Mr Jones, is to wait for slack water. First, however, you have to calculate when that is, and get it right. Get it wrong, and you may be about to find out how reliable your boat's engine and steering gear are.