I used the Altar of the Sea in Version 1.11.6 and upon placing all four items the world shut down. I updated to the latest version of Apotheosis (1.12.2 as of right now) but the world continues to shut down immediately after it seems to finish loading. I also tried switching Forge Versions from the one I had been using to the latest version for 12.2. I have played with Apotheosis for a while and used the Altar of the Sea before, the client also loads fine so I don't believe it was a compatibility problem. Here is a crash report I got from VanillaFix, let me know if additional information is needed. This mod is pretty cool and I thank you for updating it constantly, which is why I'm not worried about the crash.

Well, thats weird, whatever item you've put on the altar had a null enchantment on it, should be a quick fix to check for that.
Anyway, in the meantime, you can enable removeErroringTileEntities in forge.cfg so your world is usable (this will delete that altar, so just note that)

Thank you, enabling it removed the items in the Altar but the block itself remained. Is there a way to know within the game whether an enchantment would be null? I don't think any of the enchantments were something out of the ordinary or from a different mod. Should I close the report since the issue was technically fixed?

Two Enchantment books (Fire Aspect 1 and Power 1) , a golden sword (Sweeping Edge 2), and an iron sword (Smite 1).

What items had you put on the altar?