Vish Vishvanath was one of nine people travelling on the British-flagged catamaran Irene when it was intercepted.
The freelance photo-journalist from Twickenham, south west London, said the vessel had been "hijacked" in international waters and surrounded by about eight boats whose crew trained machine guns on them.
The armed Israeli authorities who came on board took down the boat's British flag, he said, "which is something you're not supposed to do".
The 36-year-old was one of two Britons on board the yacht. The other was Irene's captain, Glyn Secker.
Both men were detained in Israel prior to deportation.
Arriving at Luton Airport this morning, Mr Vishvanath said he had been "almost strip-searched" and that the Israeli authorities had confiscated all his camera gear and his phone.
The group set sail earlier this week from Turkish-controlled Northern Cyprus, with a consignment of medicines, toys and water purifiers.
But the catamaran was seized yesterday around 20 miles from Gaza by Israeli naval forces, with nine people and aid for the territory on board.
Mr Vishvanath said: "We had some intelligence that we wouldn't be intercepted until we got to the edge of the Israeli territorial waters, which would have been yesterday around noon.
"We had a decent night's sleep but at about 9am or 10am we saw a frigate and a gun ship.
"The frigate followed us for quite a way. Then about another four speedboats appeared and another gun ship. In total there were about eight boats."
The boats approached the British yacht and told it to stop, he said.
"They said if we acted calmly no one would get hurt," he added.
But the captain ignored their warnings to turn back.
One of two former Israeli Defence Forces (IDF) members on board Irene repeatedly read out a statement telling the naval forces that "the occupation is illegal", Mr Vishvanath said.
He went on: "All the passengers and crew were in agreement it would be positive resistance. We weren't going to do what they told us, they would have to do it themselves.
"The captain didn't give up the helm easily."
Men who he believed were special forces commandos then boarded the boat and Tasered one of the ex-IDF men, who was putting up a struggle, he said.
"I saw both men get dragged into a support boat and taken away," the photo-journalist said.
"About three commandos ambushed me and took all my camera gear. They confiscated my cell phone because it had a camera on it.
"There was a lot of resistance - people just weren't going to play ball - but there was no violence," he added.
The captain then cut the fuel lines so the Israeli authorities had to tow the boat for two to three hours to the Israeli port of Ashdod, he said.
They removed the yacht's satellite phones and shut down all its communications, he added.
According to a spokeswoman for the activists, the non-Israelis on board were transported to an immigration centre in Israel, where they were temporarily detained.
Five Israelis, one US and one German citizen were also on the boat.
Mr Vishvanath said he was not an activist but was there to document events.
He said he was the only non-Jew on board and that all had been prepared to be intercepted by the Israeli authorities.
"The others were pointing out that it's not in their name," he said. "That all Zionists are Jews but not all Jews are Zionists."
A Holocaust survivor among the group lectured the commandos at length for about four hours, he added.
"They were engaging in conversation. They were friendly enough," he said.
Israeli military officials said Irene was taken over after the captain ignored two warnings to turn back.
"No violence of any kind was used," they said.
London-based activist group Jews for Justice for Palestinians organised the mission - called Jewish Boat To Gaza - and said its last contact with Mr Secker was at 10.37am British time yesterday.
The incident comes four months after a deadly Israeli raid on a Turkish-led international flotilla aiming to break the blockade. Israeli commandos killed nine Turkish activists sparking a major international incident.
Israel imposed its blockade on Gaza three years ago in a bid to stop Islamic militant Hamas from building up an arsenal of weapons, but the move has proved inflammatory ever since.
Pro-Palestinian activists have sailed a string of blockade-busting boats to the coastal strip over the past two years, although few have reached Gaza.
