Leaving Poto Tano is a bit of a mixed bag, especially in the afternoon and evening. Basically, if you are arriving on the ferry from Lombok, try to buy a through-ticket to your final destination from wherever you are leaving in Lombok (Mataram, for example). This is because arriving in Poto Tano in the afternoon or evening without onwards transport can see you stranded or paying a lot to charter a bemo or car to your intended destination.
If you're heading south, the destinations can be a little confusing, as most buses go to other towns near the beach from where you need to arrange onwards transport (ojek or bemo) to the beach. The primary destinations are Kertasari (get to Taliwang), Jelenga (get to Jereweh) or Maluk and Sekongkang (get to Maluk).
During the day, buses between Sumbewa Besar and Taliwang, Jereweh and Maluk stop at Poto Tano before heading onwards south to their destination (and in the reverse).
In the case of Taliwang, these buses should be common till mid afternoon, after which their frequency drops right off.
To Jereweh and Maluk though, there are only two buses a day (both leave Sumbewa Besar "in the morning" - we couldn't get a more accurate timing than that!). If you miss both, you'll need to get a bus from Poto Tano to Taliwang and arrange onwards transport from there, but the ojeks and bemos there can be rapacious. Count on 50,000 for an ojek or 120,000 to charter a bemo from Taliwang to Jelenga -- and don't be surprised when the bemo driver starts off asking for 500,000 rupiah.
The best way to avoid this is to time your crossing for a morning ferry as this will give you the best chance of a bus onwards to the south. If you arrive late at Poto Tano and there are no buses south, your options are limited and expensive unless you are able to share transport.
A bemo in Poto Tano quoted us 500,000 rupiah to Maluk and 300,000 to Jereweh. On the other hand, if you can get a seat in a shared bemo from Poto Tano to Taliwang, it should cost just 20,000 rupiah.
Ferries between Poto Tano and Labuan Lombok run hourly 24 hours a day and the trip takes about 1.5 hours depending on weather. The fare for a walk-on passenger is 12,800 rupiah, 26,500 rupiah with a bicycle and 51,500 rupiah with a scooter (under 500cc). A family car will set you back 445,000 rupiah.
Note that if you are heading to Maluk, you can get a ferry from Lombok to Benete, which is just a couple of kilometres from Maluk. The ferry leaves Kayangan, Lombok (beside the main ferry terminal) twice a day at 10:00 and 16:15, costs 135,000 rupiah for an adult, 75,000 for a child and takes one hour and fifteen minutes. If you are headed to Maluk, take this boat rather than coming to Poto Tano and getting buses and ojeks down.