It seems to me that there is likely a difference between the legality of 1) the naval blockade, and 2) the legality of the blockade/embargo as a whole (which arguably comprises, in its present form, collective punishment of the civilian population of Gaza, and in its sweeping extent is disproportionate to any military advantage gained).

Some of this hinges on whether the Gaza is still considered "occupied territory." Israeli ground forces and settlers withdrew in 2005, but the IDF has full control of Gaza's air and sea space, most of its borders, and can enter the much of the area at will. The US, EU, UN, and ICRC continue to describe Gaza as "occupied territory" (which has clear legal implications under the 4th Geneva Convention). Israel argues that Gaza is a hostile, not occupied, territory.

Final note: Gaza has no seaport. It has a small fishing harbour/breakwater, and that's it (picture below, taken on my last visit).