There are plenty of vendors selling cheap seafood on the beach of Sihanoukville specifically in Serendipity Beach and Occheuteal Beach. Depending on your bargaining skills, you can really score cheap deals with some vendors.
It might help also if you’re using Riel instead of USD when bargaining with them as it can get confusing sometimes.
Mantis Shrimps: You can buy this for as low as 10,000 Riel ($2.5) for 5 to 7 Pcs
Grilled Squids: You can buy this for as low as 8,000 Riel ($2) for 8 to 10 Pcs, depending on size.
Variety of Cooked Food: Grilled and Fried Fishes, vegetables Etc.
Fruits for deserts
There are also crabs and shrimps being sold by these vendors. Alternatively, there are also restaurants selling seafood on the beach but for a higher cost.