How is the sea different from the bay?

Seas and bays are minor water bodies after the World Ocean. Their main differences:
Water exchange. The bay, as part of another body of water, has a free water exchange with the main water unit. The sea can be fenced off from the ocean by physical barriers and, accordingly, have limited water exchange.
Belonging to a reservoir. A bay is part of any large body of water (river, ocean, sea, lake). The sea, however, can only be part of the ocean to the basin of which it belongs.
Salinity. The sea can be weak or strong, but it is always salty. The bay can be either salty or fresh, depending on its affiliation to the reservoir.
Individuality. Due to the fact that the sea is separated from the ocean by land, it can have an individual hydrological regime. There is free water exchange in the bay and therefore its hydrological regime depends on the reservoir where it is located.
Border. The sea does not have to be in contact with land (Sargasso Sea). The dry land forms the bay.

The bay differs from the sea in water exchange, boundaries, belonging to a water body, salinity and possible individuality in hydrological regimes. The main feature of the bay is its coastline. And the sea, unlike the gulf, is never fresh.