A long "safety sausage" or SMB, such as divers use, can help a lot. Preferably bright orange/red or maybe yellow. Pre-inflate it and tow it on a short line - it will give virtually no drag. Then if a boat approaches simply wave it in the air. It is very visible, which otherwise you certainly are not. But keep watching the boat and be prepared to duck dive to the bottom. Don't try swimming sideways as you won't be able to move fast enough.