“Quick, I need the camera. There’s a jelly.”

Junior’s enthusiasm takes me aback. He has a healthy aversion to getting close to jellyfish. We have already changed course many times on today’s high tide swim to avoid the trailing tentacles of compass jellyfish.

These common summer visitors have striking brown V-shaped markings around their edges, like the points of a compass. Although their sting is rarely serious, somewhere in the region of a stinging nettle in strength, it isn’t much fun if you swim face-first into one as I have done on a few occasions.

Some other species we have seen this week, like the moon jellyfish and crystal jellyfish, are harmless but today only the compass jellies are out.

Incredible numbers of sand eels fill the water in every direction, flashing silver as they turn, before melding into the green sea. Junior notices a small spotted catshark (Scyliorhinus canicula) – also known as dogfish – swimming through a rocky gully beneath us. Alongside submerged rocks, several species of wrasse flit among the kelp.

I unclip the camera from the safety float and pass it to Junior who is pointing excitedly at something I can’t see.

I dip under the water and look at where he’s pointing. Still nothing. I bob up for air and try again.

This time I see something much smaller than I was expecting, or half see it – it’s mostly transparent with just the faintest pink hue.

“Is it a comb jelly?” Junior asks. This is the first one he’s seen and much excitement ensues as he tries to photograph a barely visible tiny swimming thing while holding his breath and floating in water 5 metres deep.

Mostly we just enjoy the incredible coloured light show this Beroe cucumis comb jelly is putting on for us. The iridescent disco-light effect is created by lines of beating hair-like cilia (the combs) that run the length of the comb jelly’s body.

This species looks like a simple hollow tube or sack, but it is an efficient predator, known to feed on other comb jellies.

How Junior spotted this little speck in the ocean, I have no idea. We look around for more but find none.

Eventually we have to head back to shore, drifting over all of our familiar rock pools on the way. Hermit crabs and netted dog whelks are out in force and as we near the beach, we see shannies basking on sunny rocks in the shallows.

This might not be rock pooling in the usual sense, swimming on a high tide gives us a whole new perspective on life here. You don’t need to be a billionaire to become weightless and take a soundless flight over the rock pools. There is no better way to see how this environment looks for most of each day, when the wider ocean and the shore cross over and become one.

Swimming in the sea in Cornwall is a wonderful experience but is very different from swimming in a pool and can be dangerous. Always consider the conditions and stay well within your limits. Check the weather, tides and currents, enter the water slowly and adjust to the temperature. Choose a lifeguarded beach if possible and a place where you know how to safely enter and exit the water. Swim alongside the shore. A tow float makes you more visible and beach shoes can protect you from weever fish and sharp rocks. Don’t swim alone and let someone know where you are. In any emergency at sea or on the shore, call the Coastguard on 999.

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