I have been trying to find my grandfather and I might have found his service record from WWI but I don't fully understand it. I was wondering if anyone can explain to me what all the entries mean especially what the "E form to AG. 9" was and what was the "Sea Draft Battalion."

Class A/245
28.3.16 Mobilized for immediate service.
8.4.16. Attached to Sea Draft Battalion. (MZT/581)
4.6.16 On list, retained in C.P. Depot, ending RND 5.5.16 E form to AG.9, 28.6.16. (Seaman Ratings Battn BMT/944)

I appreciate anyone's help thank you!

I cannot interpret all of it, but I can suggest:

RND - Royal Naval Division

CP - Crystal Palace

For details of the RN Division, see:

http://www.nationalarchives.gov.uk/documentsonline/royalnavaldivision.asp#interpret

and also:

http://freespace.virgin.net/jack.clegg/RND.htm

..sorry, the National Archives link should have been:

http://www.nationalarchives.gov.uk/documentsonline/royalnavaldivision.asp

Yes thank you. I checked out those sites.
Also I noticed his rating was O.S  A.B. and I don't know what that means. His service # begins with Tyneside  and a Z number if that means anything.

OS = Ordinary Seaman
AB = Able Seaman

The Z number you refer to may be his Service Number. I am guessing he was a reservist (RNR or RNVR?).  If he was RNVR, they would have had a letter designating the port division (i.e B for Bristol, M for Mersey, T for Tyne etc.  For those joining the reserves after 1914 a letter Z was added.