Ilha means island in Portuguese – this long stretch of sand gets its name from the rocky island just off-shore. The beach is at the end of a dusty road and there’s one restaurant by the car park. Close to this, the beach has a family feel with inflatables in the shape of swans and flamingos standing sentry on the sand beside the lifeguard’s raised chair. There’s little shade, so many families have brought their own beach umbrellas and the colourful tops spout like mushrooms along the shoreline. It feels friendly, non-touristy and calm, but we keep walking, looking for a quieter patch of sand. We don’t have to go far – just paces down from the inflatables, the sand is empty. We keep going till they are out of sight and it seems it’s just us on the beach.

The island, typically, doesn’t seem that far out. Most things don’t from the shore. But once I’m in the water I feel confident I can cover the distance. The channel is sheltered and there’s little wind, so I put my head down and swim. The water gets deep and dark fast and then I can’t see far below me. It’s not crystal clear, like in Greece, so instead I see a mottled green, penetrated occasionally by rays of sunlight.

I enjoy the swim, the strength in my body as it pulls through water, the weightlessness of it. Gulls flap down metres ahead and sit bobbing on the water, their heads turning sharply from side to side as they scan their surroundings.

The island comes closer and closer and I swim and swim. As I get close, I see a sign prohibiting access. I hold onto the plastic jetty instead and bob. There’s not much on the island – birds and scrub and the broken down walls of what once must have been a building. The beach looks further away than the island looked. It’s always further on the way back. One hundred metres away, snorkellers are drifting, their colourful pipes waggling in the air. I dip my face and hold my breath, trying to see life through my goggles as they do. But I can see nothing, and I don’t want to be gone too long, so I turn for the shore.

As I swim, I get really hungry. The salt, the exercise. The irrational guilt that the lifeguard is going to tell me off, that I’ve broken some rule. But I keep swimming, aiming for a green and blue umbrella – the people are sunbathing too low to the land for me to see them. I get closer and closer and my stomach growls and my arms yawn as they pull me onwards. I reach the edge and walk back breathless along the beach to eat cheese sandwiches, repair my muscles and read in the sun.

I’m raising money for the Alzheimer’s Society. Please sponsor me at http://www.justgiving.com/swimbonnieswim