It looks like a worn-down water chestnut. Water chestnut fruits are often found along the shoreline and bottom of waterways - they have very sharp spines with barbs that can cause painful wounds when stepped on. Water chestnut is an aquatic invasive plant that is native to Eurasia and Africa. It was introduced in the United States in the mid-1800's as an ornamental plant. Around 1884, water chestnut was found growing in Collins Lake near Scotia, NY. Water chestnut colonizes areas of freshwater lakes and ponds and slow-moving streams and rivers and negatively impacts aquatic ecosystems and water recreation.