Hi Anita,
The adventurous bus ride is the way to go to La Paz. It's not possible to cross borders by boat. After visiting the floating islands on Lake Titicaca in Puno (Peru) you take a bus to Copacabana in Bolivia.
While you're at Copacabana you can spend the night on Isla del Sol (see photo) on Lake Titicaca. From Copacabana it's a 4 hour bus ride to La Paz.
Best wishes, Mark