Yes, SEAL is a Scrabble word!

Letter values in Scrabble US: S 1, E 1, A 1, L 1.

Is it a valid word?

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

fastener consisting of a resinous composition that is plastic when warm; used for sealing documents and parcels and letters
a device incised to make an impression; used to secure a closing or to authenticate documents
the pelt or fur (especially the underfur) of a seal
a member of a Naval Special Warfare unit who is trained for unconventional warfare
a stamp affixed to a document (as to attest to its authenticity or to seal it)
an indication of approved or superior status
a finishing coat applied to exclude moisture
fastener that provides a tight and perfect closure
any of numerous marine mammals that come on shore to breed; chiefly of cold regions

Also: sealing wax, navy seal, seal of approval, seal off.