Israeli forces later stopped and seized the main protest boat as it approached the blockade limit at nine nautical miles, while others were said to have turned back.
Access in and out of the Gaza Strip by land and sea for its two million inhabitants is strictly controlled, but what is the effect of the blockade?
Restrictions on the thin coastal strip – just 25 miles long and an average six miles wide – were ramped up when Hamas won elections in 2006 and took control of Gaza the following June after a battle against Fatah.
Hamas refuses to accept an Israeli state, is regarded as a terror organisation by many countries – including the US, and for years has carried out attacks such as firing mortar shells and suicide bombings.
Israel says the measures are absolutely necessary for its own protection – but the effects have virtually crippled the Gaza economy and led to it being compared to an open prison.
Some 80% of its citizens need humanitarian aid to survive and the UN says its people are “locked in, denied free access to the remainder of the territory and the outside world”.
The problems have been made worse by Egypt, to Gaza’s south, tightly restricting people crossing into its territory at Rafah, only opening it for short and unpredictable periods.
Thousands of young people in Gaza therefore find it very difficult to leave to find work or study abroad and families are unable to see their loved ones.
The Oxfam charity says 96% of water is undrinkable and 47% of people don’t have enough food.
There are also regular power cuts. A major power plant was destroyed in the 2014 war and Gaza is at the mercy of Israel, Egypt and the rival Palestinian Authority for its electricity.
Last year, it was only available for a few hours a day.
As well as affecting people’s everyday life, that poses obvious problems for businesses, construction and healthcare.
The open “prison” analogy cited by critics of the blockade is backed by the UN, which says only a “small minority” of Gazans are able to get exit permits.
It says they are “primarily patients, business people and staff of international organisations”.
But sick people seeking treatment often have to wait a long time to get their permit, or may get turned down altogether.
Restrictions on imports – such as certain chemicals and fertilisers, building materials and technology – have also stunted the economy, for example slowing the construction of new homes.
Israel deems these products “dual use”, because they could also be used by Hamas for terrorism or boosting military capabilities like making bombs or building bunkers.
A six-nautical-mile restriction on Gaza’s fishermen – enforced by armed Israeli patrols – makes it difficult for them to make a living, while most farmers cannot export their crops.
Gaza could be unliveable by 2020 if things keep deteriorating, a UN report reiterated last year.
It said life was getting “more and more wretched” for its citizens – especially as its population continues to quickly increase.
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However the blockade shows no signs of ending – especially with Hamas still in power, shells fired in its direction and violent protests at its border.
The country is bolstering the physical barriers further and is even building a “new and impenetrable” barbed wire-topped barrier a few miles north of Gaza to “prevent the possibility of penetrating Israel by sea”.