They held most of gaza hostage, blocking their access to international waters off gaza's own coast, based on the actions of a much smaller subset of those people. That seems about the most classical example of holding hostage as it gets.
But it was closed after a civil war within Gaza where Hamas took over circa 2007, not in response to a war with "other" countries. The blockade has been in places for nearly 20 years.
I think if Israel believes that weapons can be smuggled via sea, which is reasonable given the smuggling via Sinai and Rafah crossing, then they took the rational step of mitigation this risk.
Gazans could smuggle in arms, ergo refugees can't escape out into international waters towards whoever might receive them?
That doesn't make sense, it seems as if they're held hostage by Israel forced to stay in the very land where their own terrorist government might impress them into servitude towards use against Israel.