Important is related to Crucial. Crucial is a synonym of Significant. Significant is a synonym of Relevant. 
Water is related to Swimming. Stream can be used for Swimming. Stream is the opposite of River. 
Important <--RelatedTo--> Crucial <--Synonym--> Significant <--Synonym--> Relevant 
Water <--RelatedTo--> Swimming <--UsedFor-- Stream <--Antonym--> River 