sometimes sea lantern block doesn't have light, but you can relight it if you another light source and remove it.
MC-65070
sea lantern has no light
Resolved
Duplicate
0
mohammed alhosani
(Unassigned)
Unconfirmed
Minecraft 14w31a, Minecraft 14w33c, Minecraft 14w34a, Minecraft 14w34b, Minecraft 14w34c, Minecraft 14w34d, Minecraft 1.8-pre1, Minecraft 1.8-pre2, Minecraft 1.8-pre3
Retrieved
This issue is not yet resolved, the bug was not with the blocks failing to update fast enough (which the duplicates tracker says it seems to be); it's the fact that sea lanterns will frequently remain unlit unless they receive a lighting update themselves.