Yes, JELLYFISHES is a Scrabble word!

Letter values in Scrabble US: J 8, E 1, L 1, L 1, Y 4, F 4, I 1, S 1, H 4, E 1, S 1.

Is it a valid word?

JELLYFISHES is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

large siphonophore having a bladderlike float and stinging tentacles
any of numerous usually marine and free-swimming coelenterates that constitute the sexually reproductive forms of hydrozoans and scyphozoans

Also: portuguese man-of-war, man-of-war.