I see online that one comes with LakeVu or the Blue Chart G2. You definately need the G2 for the ocean. I just bought the 840XS and it came preloaded with the Blue Charts and they are excellent. You get navigational and fishing charts. Very handy stuff. That's too bad it you didn't get the one with it preloaded. May try to exchange it, LakeVU does nothing for you.
I've got the VUSO25R and the charts are pretty good. There are still a number of rocks that aren't marked on it that I have found though. I don't think they would be marked on any Garmin chart though, they aren't on the NOAA charts either. Its been pretty spot on in my shrimping area though.