Note: If you're referring to things that they've done within the last few days, disregard this comment as this comment was written regarding the way things were before the recent attacksI think it's more nuanced than just them trying to "prevent clean water and food and medicine from crossing the border." In the region, water is scarce and important for both sides. Israel does allow "clean water" into the region, the complaint is that Israel government allows their own people take too much of it (from the perspective of Palestinian farmers). In the American west we have the exact same problem. Farmers higher upstream of the Colorado river want enough water for all their crops, but this would deprive the farmers further west from having enough. There are intense and heated (political) battles between the regions, with both regions feeling like they are getting shortchanged.
With food, medicine etc, there's concern that those supplies are going to enemy combatants/terrorists (from the Israeli perspective).
This is one of those situations where there aren't really any clear/easy answers.