While the waves and wind were going in the right direction, the size of the swell made swimming a real challenge. I completed 9.72km, much less than I've been doing in the past couple of weeks, but still enough to keep me a little ahead of schedule.

Conditions are due to worsen overnight, though, and we don't get know whether we'll be able to swim tomorrow.

Stay tuned!

For today's swim, we were joined by Sir Simon McDonald, the head of the UK Diplomatic Service.

I first met Sir Simon earlier this year on his tour of South Africa. We spent an afternoon walking along a beach in Cape Town, where I explained the enormous changes I have seen in our oceans over the past 30 years, especially from plastic pollution.

I am now finding it all over the world, even in the most remote parts of our planet. And it is having a devastating impact on life in our oceans.

On his return to the London, Sir Simon immediately ordered an audit of all the single use plastic in the Foreign and Commonwealth Office (FCO). When you think that the UK has embassies and high commissions in almost every country of the world, and hosts many functions, you'll appreciate the extent of the problem.

The results of the audit made it clear that he needed to act quickly to end single use plastics in the FCO, including in every UK embassy around the world and in all their supply chains. This single decision is now having a major impact both within government and with suppliers around the world.

When Sir Simon came on board Aquila this morning, it was the sunniest and calmest weather day we have had in weeks, so we picked up from where we had left off. We grappled with what further action the UK could take on single use plastics, and the other two major issues of climate change and over-fishing.

I urged him to accelerate the protection of the waters around British Overseas Territories, especially those around South Georgia and the South Sandwich Islands, which are one of the most important biodiversity hot spots on the planet. Currently less than 2% of this area is fully protected.

When I did finally start today's swim, I was acutely aware of Sir Simon watching me so I managed to swim nearly 15km. That made up for the distance lost due to yesterday's bad weather! There were also two milestones crossed with today's swim - I've now swum over 400km and done the equivalent of 12 back-to-back Channel crossings.

On the motor back into Brighton, Sir Simon and I discussed how change doesn't always start with meeting the heads of government departments.

We each have our own personal networks of friends, family, colleagues and our local community that we can urge to stop using single use plastics and think about how we can reduce our own impact on climate change.

Every single one of us is a multiplier. If you can convince just a few people to stop using single use plastics, for example, they can each convince another few in turn - so the message spreads and real change begins.

We owe it to our blue planet - and to our children.

9.2 miles (14.8km) - Total 255.8 miles (411.6km)