Looking for efficient and economic methods to keep the sea pollution-free

Coastal touristic cities have already invested time and money to remove trash from their beaches.  

We are searching for some efficient and economic methods for collecting trash from the sea to keep it  pollution free.  

Furthermore, we are interested in knowing if it is more efficient and economical to collect the trash from the beach or at the sea? Does anyone know any study that made the comparison of these two?  

Thanks to anyone who can help us or has any advice!