Twelve-year-old Sabah Abu Ghanim drags her board through the water as the sinking sun glints on the eastern Mediterranean. The wind has got up a little, and she is hoping the surf will follow – enough, at least, for her to ride the waves.

"I feel the sea belongs to me," says the Gaza surfer. "When I'm in the sea I feel content and happy."

Gaza's 25 miles of Mediterranean coastline are a magnet for a population with few forms of entertainment and a pressing need to escape the drudgery of life under blockade. A small but growing band of surfers use a variety of makeshift boards plus a few sent by surfer activists through the heavily controlled crossings from Israel.

"We have a problem getting equipment in, but these guys try to do the best for themselves," says Mahfouz Kabariti, president of the Palestine Sailing and Surfing Federation. "Always people are suffering from the siege, so they need some space to feel normal. This is good for their mental health."

Sabah was taught by her lifeguard father to use their 22-year-old surfboard, which is shared between friends and family. The first time, she says, "I put myself in God's hands. I said my last prayers. And I surfed a very high wave."

Now she studies TV and the internet to improve her technique. She, in turn, coaches others. "When my friends see me surfing they are very proud. They like it and ask me to teach them. They become surfers like me. Not exactly like me, not quite as good as me."

Gaza's beaches are along one of the few undeveloped stretches of Mediterranean coastline. There are no tourists to fill the handful of seafront hotels. A few beach cafes, some run by Hamas, attract locals outside Ramadan.

Ramshackle lifeguard towers are stationed periodically along the beach, but there are no signs warning swimmers of the greatest hazard – the sewage in the water. Up to 80m litres of sewage is dumped in the sea every day, causing diarrhoea and skin complaints among those who swallow the water.

Gaza's four sewage treatment plants cannot cope with the growing population, according to Ewash, a consortium of international and local NGOs. Israel's continued blockade prevents materials needed for maintaining and upgrading the plants from reaching Gaza, it says.

"People should be warned about swimming in areas close to sewage outlets," says Ghada Snunu of Ewash. "But it's not easy to tell people to stop swimming. The beach is the only recreation for the majority of Gazans."

After getting sick, Sabah says she has has avoided the most contaminated areas. But most people prefer to risk illness than give up one of their few pleasures. Her father, Rajab Aby Ghanim, 37, a self-taught surfer, is proud of his daughter's prowess and is planning to introduce eight-year-old Saja, Sabah's sister, to the joys of surfing. But, he says, "I have many problems with my daughters surfing. Many people criticise me. I asked my two older daughters to stop because of the community."

Sabah sometimes senses disapproval of her activities from some conservative Gazans. "There is a difference [between boys and girls]. When we are swimming in the sea and men see us, they are very surprised. They tell us to get out."

"When I am older, my society refuses to allow me to surf. It's shameful. I will keep surfing until then, and then I will have to stop. I will be sad," she says.

Once, she says, her 16-year-old sister to come to the beach to watch her surf. "I found her sad. I said, 'You keep wishing to go back to the old days because then you could surf and swim.' She said, 'I wish those days would return.'"

Her mother and aunts sometimes come to the beach to swim "if no one is around. But if others start to arrive, they get out and go home. We don't want people to talk about us."

But, for now, Gaza's surfer girl is riding the waves. "People are proud of us. They say, 'This is the first time we saw a girl who knows how to surf.'"