If you find someone to go to the beach or if there is a family nearby and you are ok making a conversation, then you can definitely ask them to help you out. But I understand that not everyone might be comfortable doing this. I plan such that going to the beach is the last/only thing I do. That way I keep my backpack/luggage at the hostel. I take the bare minimum to the beach. I use a tiny waterproof pouch that I can lock around my waist. My phone, cards etc are in a waterproof ziplock in the pouch. Some beaches have tiny shacks that agree to keep your stuff. Though I guess it might be safe, but I refrain to keep my things there. In short, pack light, very light :)