Transturin runs catamarans on Lake Titicaca — the slower, more comfortable alternative to Crillon's hydrofoils — and sells them inside longer circuits that string La Paz, the lake, Uyuni and Cusco together with the internal flights booked.
Their published four-day Cusco-to-Cusco itinerary is a useful thing to look at even if you do not buy it: it shows you what the lake and the salt flat cost in days when you do them back to back.
The office is out in Achumani. Landline and one mobile, neither of them advertised as WhatsApp.