Sayyiduna Abu al-Darda' (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "Whoever stays by the sea, hoping for reward and with a sincere intention, to protect the Muslims, Allah Almighty records for him a good deed for every drop in the sea."
Al-Tabarani recorded it; its chain contains Yusuf bin al-Safr, who is matruk (abandoned), and the chain is munqati' (broken).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الجهاد / 9491