The name of the supreme god of the sea (I do not mean Neptune) is Slavic?

The answer to this question depends on which Slavic culture you are referring to. In some Slavic cultures, there is no supreme god of the sea, while in others, there are multiple deities associated with the sea.

In the Slavic mythology of the Baltic Sea region, the god of the sea is called Jūrmala, who is the ruler of the oceans and the protector of sailors. Jūrmala is said to be a powerful and wise god who is capable of both granting and punishing mortals.

In the Slavic mythology of the Carpathian region, the god of the sea is called Vodyanoy, who is a powerful and unpredictable spirit. Vodyanoy is said to be the ruler of the underwater realm and the protector of fishermen. He is said to be a shape-shifter who can transform into a frog, a snake, or a human.

In the Slavic mythology of the Black Sea region, the god of the sea is called Pontos, who is the ruler of the ocean and the protector of sailors. Pontos is said to be a powerful god who can grant mortals with wealth and prosperity or punish them with storms and floods.

In the Slavic mythology of the North Sea region, the god of the sea is called Veles, who is the ruler of the oceans and the protector of fishermen. Veles is said to be a powerful god who can grant mortals with wealth and prosperity or punish them with storms and floods.

Overall, there is no single supreme god of the sea in Slavic mythology, as different regions have different gods associated with the sea.