How to set/fix initial spawn on custom survival map

So I have made my own survival map and it is an underwater glass dome, it's a little away from where I first spawned when creating the file for the first time. I tried using single player commands to setspawn to a specific set of coordinates, but every time I die I spawn above the dome in the water, which is no good for players. Can anyone tell me why this is happening and if there is a fix or mod that will let me set a spawn point inside the dome so I can then upload the save and post it blah blah.

Thankyou in advance

**Edit
This has NOTHING to do with beds so no suggestion about that