This is Tiquina Strait. One needs to cross this strait if one wants to go to Copacabana City, to the Sun and Moon Islands and to other islands.
One takes a minibus from La Paz to Tiquina and after about two hours one is in Tiquina. Then, one needs to buy a ticket to take a boat to cross the strait.
On the other border, one may take a taxi or a minibus to go to Copacabana. For me this is the best way to go to Copacabana.
One thing to take into account: one may cross the strait up to 21:00, but there are times when it is closed at 5:00 in the afternoon, especially in windy or rainy times.
One day, I crossed it at 20:00 and it was really cold but as a gift I had the pleasure to see the stars shining beautifully.