What are the disadvantages of building a prison surrounded by a big water body?

There are a few things to consider. Keep in mind that all of these vary by degrees.

• Access. Most places “surrounded by water” have a bridge or tunnel into them. This works better than boat or air delivery as prison populations consume a lot of resources over a short period of time. Each of these items has different pros and cons for rate of transport and security. I can’t go too much into each as they are so highly dependent on their environment and layout.
• The water itself. Is it naturally occurring or an artificial construct? Moving water or stagnant? You are likely going to need to maintain this water to some degree. Natural flowing sources are the easiest for maintenance but likely come with erosion concerns. Slow or still waters could be a pest problem if not looked after.
• How deep is it and where is the water table? This may be a source of water for the prison, or not. Also, the depth of the water and the water table can make any underground portions of the construction more difficult.
• How close is your prison to the water? This creates issues of whether the prison itself needs operate as a dam.
• Security. Everyone’s big question with regards to prisons. Yes, a body of water creates a barrier around your prison that is, in theory, easier to monitor. Access becomes more controlled. The surroundings, easier to survey. But, most structures require an interface zone between the structure itself and the body of water. This can be inches or it can be miles. Is it paved or natural? All of a sudden you can lose the advantages you had by creating blind spots surrounding your prison.
• Political pressures. Keep in mind that prisons might sound like a good idea, but you probably don’t want to live next to or near one. And neither does anyone else. Like most large projects it will become a series of compromises. Where you are allowed to build may not be ideal. Interface zones or screening may conflict with the level of security you are trying to achieve. Local or state issues of safety, such as fire or natural disaster concerns may be imposed on your design despite your intention of level of security.

I could keep going but hopefully, I covered your basic concerns.