I don't know if this is a bug, my imagination or if it was added on purpose but me and my friend were playing on a 1.14 server (made by us) and when we went in "gamemode spectator" we saw that the underworld (from grass till the bedrock layer) was much smaller then how it used to be. After to make sure I went on other versions such as 1.7.10, 1.8, 1.12.2 and saw the same thing. Basically, you need to dig just some blocks under the ocean and you will find bedrock.

Again, I don't know if this is a bug or just my imagination but as far as I remember the underworld was much bigger.