This update has been in development for some time now but I was pleasantly surprised by what they've added so far...
- Turtles as well as a turtle helmet and potion that both are pretty useful
- Phantoms, hostile flying mobs that attack you if you haven't slept in a bed for too many nights
- Drowned, water variant zombies which have a chance to carry...
- Tridents, can be used as a spear as well. I recommend Mevans looks at the code they seem to be pretty effective when it comes to not bugging out although a little OP at this time
- New swimming animation, looks kind of weird and buggy with the lighting but certainly a welcome feature, you can still swim the old way but if you try to sprint you will go into the new mode
- Kelp, sea grass and coral. The ocean looks 50% prettier now being less just a clusterfuck of gravel and clay
- Coral sub-biome, looks really nice
- Fish mobs, some of them swim in groups I think this would be a welcome addition to the LOTR mod for more variation and also movement AI
- Shipwrecks, like an underwater dungeon, less significant then the ocean monument, probably slightly more common. About the same magnitude as the Desert Temple. Loot.
- Items float up in water
- Conduit, mysterious block which doesn't have a purpose yet, made of rare and new resources from the ocean
- Ocean ruins, really nice looking, have about 100 different variants and composed of random amounts of different types of blocks, meh loot but lots of it
- Intentional underwater ravines, look pretty damn nice, some have magma and obsidian at the bottom, also be careful because magma now drags you down in the water and can lead to drowning if you're above it, is this a legit thing in real life to do with thermodynamics? I have no idea
- Buried treasure, you find the maps in ship wrecks and the like, pretty damn good loot.
The new update has some really nice looking aesthetics and I would recommend that Mevans add many of the features to the mod. I was surprised by the amount of content but it turns out they were planning to do this over two but decided to actually work towards a decent sized update.