The Titanic was unsinkable. The Costa Concordia would never capsize… So yes, ships can sink.
However, as we normally sail in the calm and sheltered waters of the Tagus River and not in the open sea, this probability is even lower. The risk of the boat capsizing or sinking is practically nil. Our sailboats and catamarans are built with keels and/or hulls that ensure high stability and are designed to withstand sea conditions.