Their reason was that they were afraid Gazans would leave via the same boats, and no one wants Gazans to flee, except Gazans.
Unlike Israel, which welcomed the plan (and in fact had been pushing since early November), there have been no public statements by Hamas or Fatah about the planned US pier and the aid corridor being implemented now.
Ultra Palestine went and asked the terror groups their reactions. And the answers are interesting.
One anonymous "factional source" said that Hamas stipulated that only specific groups and Hamas allies be able to receive the aid from the ships. This includes the Gaza city municipality, which is run by Hamas, in addition to international relief institutions, and the Emirati side that sponsored the project. He also warned that US troops on the pier could easily be attacked.
PFLP official Haitham Abdo warned against the possibility that Gazans could leave via the sea, as well as expressing reservations over the speed of its implementation.
Islamic Jihad remained silent about the plan, as has the PA government. There have been no articles about it in official PA media. Perhaps because they don't want to publicly oppose the US.
MEMRI's Yigal Carmon loves the plan, calling it "the beginning of the end of the war." He says Qatar and Iran are against the project, since Qatar is not involved and it has used its near monopoly over Gaza aid to make itself appear indispensable. He also claims Egypt is against the move because it is relying on bribes at the Rafah crossing. But he is excited that the UAE and Jordan are on board, thinking that they will be heavily involved in distribution, instead of Qatar.