There are two ferry crossings at Silistra/Calarasi.
The new one that Bulgariabill has kindly posted the timetable for and the old one that runs from dawn until dusk everyday every 30 mins or so.
The new one is situated a few kilometres west of the city near to the town/village of Aydemir.
The older and more reliable one is to be found just across the land border point in Silistra.
If you want to catch the more regular one, which is free for foot passengers, drive into the city and follow signs to Romania/Bucharest.
If you'd rather wait for the more modern one, which seems to only run when it's full, stay on the bypass and follow the signs depicting a ferryboat transporting cars.
Both ferries arrive and depart from virtually the same place on the Romanian side. A few kilometres from Calarasi.
There is a third option for daytrippers of taking a water taxi from the Drustar hotel directly to the centre of Calarasi.
If anyone would like more precise directions please just send me a PM.