A huge worker in overalls towering above the Bund, holding a red flag of the Shanghai Revolutionary Committee (上海市革命委员会) and with the Selected Works of Mao Zedong (毛泽东选集) in his breast pocket.
The two small figures in the water must be Cao Diqiu (曹荻秋) and Chen Pixian (陈丕显), the two leaders of the Shanghai Municipal Committee.
The poster is 'signed' by three organizations:
上海工人革命造反总司令部 (Shanghai Workers' Revolutionary Rebel General Headquarters)
上海市灭资兴无铲修挺进军革命委员会 (Shanghai Municipal ??? Revolutionary Committee)
上海市房地局工人革命造反总指揮部 (Shanghai Real Estate Bureau Workers' Revolutionary Rebel Headquarters)