Conversation so far:
How did Dennis die?
he fatally drowned in Marina del Rey.
When did he die?
December 28, 1983.
Was he with the band when he died?
Dennis was given an ultimatum after his last performance in November 1983 to check into rehab for his alcohol problems or be banned from performing live with them.
Did he check into rehab?
Dennis checked into rehab for his chance to get sober, but on December 28, 1983, he fatally drowned.
What did the band do after his death?
From 1980 through 1982, the Beach Boys and The Grass Roots performed Independence Day concerts at the National Mall in Washington, D.C..

Question: Did they perform anywhere else during that time frame?
Self-contained rewrite: Did The Beach Boys band perform anywhere else other than Independence Day concerts at the National Mall in Washington, D.C. during that time frame of 1980 through 1982?