That's not true. There are over 10,000 wells in Gaza. Hamas controls who is allowed to dig wells in Gaza [0] – Israel lacks effective control over the territory of Gaza so has no ability to stop anyone from digging a well there.
The problem Gaza has, is a lot of the water from the wells is undrinkable, in part due to intrusion of seawater into the aquifer. Israel extracts a lot of water from the same aquifer further inland, which is argued worsens the seawater intrusion problem. However, desalination is used to deal with this problem, and international donors have spent significant sums on providing desalination equipment to communities in Gaza. That requires electricity (of which there is not always a reliable supply), and parts and consumables for repair and maintenance (for which there can sometimes be difficulties getting Israel to permit their import.)
> Fishing is illegal
According to [1]: "The total catch in Gazan waters has risen in recent years, according to the Palestinian bureau of statistics, from 2,322.9 metric tons in 2006 to 3,943.4 in 2019, when Israel expanded the permitted fishing zone to 15 nautical miles in some areas from six, but still short of the 20 nautical miles agreed to under the Oslo accords, the 1990s peace agreement that was supposed to lay the path for Palestinian statehood."
It is true that Israel has a negative impact on Gaza's fishing industry – through limiting how far out to sea Gazans are permitted to fish, and by restricting import of goods needed to repair boats, many of which are classified by the Israeli government as "dual use" technologies – but a 2019 catch of over 3900 metric tons is hardly "fishing is illegal"
[0] https://www.al-monitor.com/originals/2021/08/gazans-fear-wor...
[1] https://www.nytimes.com/2022/11/27/world/middleeast/israeli-...