Omega vintage sea master - is it real???

It’s real. The closest date can be determined by looking up the serial number on the movement (not pictured here), but this will give you some idea: https://www.omegawatches.com/en-us/watch-omega-seamaster-calendar-ck-2849

I'm not into Seamasters but based on your photos, you have a Seamaster 2849-3 (per caseback) with a caliber 503 movement. It's actually called a cross-hair dial. There should be a eight digit number on the movement that should tell you the manufacture date if you look it up on various databases. Based on the coat hanger "S" in Seamaster, it should be 1950s to early 1960s.

It also looks legit to me but the photos are very blurry.

Very real, very nice. A great heirloom to own as a reminder of your grandad. Wear it in good health!