What changes would be needed to a late model rover to enable it to function under water? What would prevent a rover from functioning under water as is?
Adapting an existing rover design to work underwater would be extremely cost prohibitive and it would be cheaper, more effective, and all around a better idea to design a whole new rover with underwater capability in mind.
In a general sense, there's a couple problems that an underwater rover needs to counter:
Pressure: Most spacecraft and rovers function in a near vacuum environment. On Earth, even at just 10 meters of water depth there is already 1 atm of pressure
Communication: Water is notoriously difficult to communicate through
Power: Sunlight doesn't penetrate water very far even without ice above it. An alternative power source to local solar panels is needed.
Corrosive environment: Salt or other chemicals in the water can cause corrosion or fatigue in all sorts of materials
If you wanted to recycle a rover such as the Curiosity rover to work underwater, basically the only think you'd keep is the RTG. Everything else would need to be replaced and redesigned.