A senior police officer told Channel 10 television that barrels packed with explosives were found on Monday on beaches south of Tel Aviv, adding that they could have come from the Hamas-ruled Gaza Strip.
The military said in a statement that "two explosive devices" were found on beaches at Ashdod and Ashkelon.
Mabhuh was found dead in Dubai after travelling from his base in the Syrian capital Damascus to buy weapons for Hamas, which has blamed Israel for the killing.
On Monday the Hamas armed wing, which Mabhuh helped found in the 1980s, vowed revenge.
"The Zionist enemy will not escape punishment for the crime it has committed, but we will determine the nature of this punishment at the appropriate time and place," Hamas spokesman Abu Obeida said.
He hinted that Hamas might reconsider its long-standing policy of launching attacks only within Israel, the occupied West Bank and Gaza Strip, all of which it hopes one day to transform into an Islamic state.
"The battle with the Zionist enemy is inside the historic borders of Palestine, and the occupation is trying to change the rules of the game... It should expect anything," he said.
Israeli media hailed Mabhuh's killing, although the government has not commented officially.
Dubai's police chief told AFP on Sunday Mabhuh's killers could be any party with an interest in his death, including Israel's Mossad foreign intelligence service.
A police commander told Channel 10 that two barrels packed with explosives had been washed ashore.
"We found today two big barrels full of explosives which washed up on beaches in Ashkelon and Ashdod, which could have come from the Gaza Strip," Allon Levavi was quoted as saying.
Beaches along the Mediterranean coast in southern Israel were declared off limits until further notice amid fears that more explosives could be found.
Channel 10 reported that each barrel was packed with dozens of kilos of explosives.
"Three days earlier, several terrorist organisations claimed responsibility for a remote controlled attack using explosives, around two kilometres (1.25 miles) off the coast of Gaza," the statement by the military said.
It said the attack was claimed by several Palestinian militant groups, including the Al-Qods Brigades of Islamic Jihad and Al-Aqsa Brigades linked to the Fatah party of Palestinian president Mahmud Abbas.
No one was hurt in the incident, a military spokesman told AFP, adding that the explosives that washed up on the beaches "could be linked" to the attack.
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