on this link by grupo ccr it is possible to check the schedule of boats going to praia de charitas by clicking on linhas, horários e tarifas on the top of the page.  

to check the return schedule, clicking on 'charitas' on the map, and selecting 'rio de janeiro' in the submenu opens the schedule, where the fares are also listed.    

this link shows praia da várzea / (praia de salinas) on a wikimapia - the link as we present it is best seen using google chrome, as it might not open correctly with other browsers.