Conclusions:
I. No Ocean is a Sea.
II. Some Plateaus are not Oceans. Statements:

From Statement I. No Sea is a River (E) + Some River are Plateaus (I) → Some Plateaus are not Seas (O*) + Some Plateaus are Oceans (I) → No conclusion. Hence conclusion I does not follow. Some Plateaus are Oceans (I) → Conversion → Some Oceans are Plateaus (I) → Probable conclusion → Some Plateaus are Oceans (O). Hence conclusion II follows. From Statement II. Only River are Seas → All Seas are River and Seas can have negative relation with anyone. Hence conclusion I follows. Only a few Plateaus are Oceans → Some Plateaus are not Oceans (O). Hence conclusion II follows. From Statement III. All Seas are Plateaus (A) + No Plateau is a River (E) → No Sea is a River (E) + Some River are Oceans (I) → Some Oceans are nor Seas (O*). Hence conclusion I does not follow. No Plateau is a River (E) + Some River are Oceans (I) → Some Oceans are not Plateaus (O*). Hence conclusion II does not follow. From Statement IV. No Ocean is an Plateau (E) + A few Plateaus are Seas (I) → Some Seas are not Oceans (O*). Hence conclusion I does not follow. No Ocean is a Plateau (E) → Conversion → Some Plateaus are not Oceans (O). Hence conclusion II follows.