What are the six characters on this seal? (Characters identified: 漢高皇帝後裔)
edited, according to info provided by droooze.

the recognised character in sequence:


５３１
６４２

character ５：後 (u+5f8c)

http://humanum.arts.cuhk.edu.hk/Lexis/lexi-mf/search.php?word=後

character ６：裔 (u+88d4)

http://humanum.arts.cuhk.edu.hk/Lexis/lexi-mf/search.php?word=%E8%A3%94

character ３：皇 (u+7687)

http://humanum.arts.cuhk.edu.hk/Lexis/lexi-mf/search.php?word=皇

character ４：帝 (u+5e1d)

http://humanum.arts.cuhk.edu.hk/Lexis/lexi-mf/search.php?word=帝

character １, the left component is 水 (u+6c34), the left one, wait :)

edited. with help from droooze, this one should be 漢 (u+6f22)

http://humanum.arts.cuhk.edu.hk/Lexis/lexi-mf/search.php?word=漢

character ２, this one would be 亙 (u+4e99)

http://humanum.arts.cuhk.edu.hk/Lexis/lexi-mf/search.php?word=亙

so, it would be read as "漢亙皇帝後裔", roughly "the descendent of emperor '漢亙帝'"

have fun :)