It is obvious. You could simply do it. One can easily arrive to that conclusion. We can without effort follow the logic here. Implementation is therefore left as an exercise for the reader. /s
Now seriously, managing sovereign borders, enclaves and the people in them is not like playing monopoly. Except for Trump, that is.
they could also protect the sea border as alternative since based on court ruling if your swim and don't cross some obstacle like fence illegally they can't deport you immediately
and as I said they are right now doing it, building the barrier into the sea, so it wont be legal to cross even there