Might take some patience and commitment to the frog, but if you throw it long enough, I've seen big monsters come and grab it in deeper open water. Cloudy days help. If it's windy, I prefer a popping frog: calmer conditions, regular frog.
Probably won't get you numbers in open water, but be ready for a big one to snatch it.
--Grapevine does have decent vegetation is some areas.