Today was a day of fantastic highs and struggling lows.

On the motor out from Plymouth to my starting point for today's swim, we were joined by a pod of 15 common dolphins who swam right up to us, riding in the bow wave between the two hulls of our catamaran, leaping and playing for a full half an hour.

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They were so close that we could hear their clicks above water and could clearly see them just below the surface before they jumped and breached.

To be so close to these animals on their terms - and for so long - was profound.

Tearing ourselves away to start the swim, I dived in and started strong.

I was joined by Cal Major, a stand up paddle-boarder who just completed a world first expedition from Land's End to John O'Groats to highlight plastic pollution.

She paddled alongside me for a couple of hours, which certainly made a change from having another swimmer.

Spending the day with Cal was a meeting of minds. We share the same core message that we want to communicate the best we can - through endurance sport.

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We must end single use plastic and better protect our oceans through marine protected areas. This is no longer our children's problem - it is ours, today.

I am now swimming in the 'neap' tides - when the current is at its absolute weakest.

This means I have virtually no help from the sea moving in the same direction as me, so achieving my goal of 10km a day is taking me longer and longer, and I need to expend more energy every day just to reach the same target.

Today it was a three hour 15 minute slog, even with Cal beside me.

My team are doing their best to cheer me on when I get to a weak moment, with our chief of staff, David, dancing on deck for the entirety of the last quarter of an hour just to get me over today's finish line.

Sometimes, though, it feels like reaching Dover in 50 days might slip through my fingers.

I'm determined to not be defeated, even when my muscles are achingly sore, my tongue has swollen up to chafe against my teeth from the sea water, and even my coffee tastes like salt.

I accepted this challenge precisely because it was so difficult to highlight the extreme urgency to act now to save our oceans.

After a difficult swim, my mood was low and I was brooding on the obstacles ahead.

As we approached Eddystone Lighthouse on our motor back to Plymouth, my team started to sing yet another rendition of what has become the song of the expedition - the Eddystone Light sea shanty.

Pulled in by their enthusiasm, I put aside my fears and embraced the moment and agreed to record my first duet with our skipper, Stephen. I think a record deal may shortly follow.

5.4 miles (10km)