Poseidon and Varuna
Similarities:
Both Poseidon and Varuna are the Gods of ocean. They are the protectors of aquatic creatures.
Aegir is the Norse God of Oceans, while Njord was the Norse god of Sea.
Nun, the Egyptian God is considered as primordial water.
Dissimilarities:
Poseidon carries the Golden Trident as his weapon, whereas Varuna carries the Pash (the weapon which is the combination of an arrow and a trident). Further, Varuna is also the God of water, but the Greek God of water is the Titan Oceaneus.