Sorry for the delay, just been tired and busy lately. Got some snapshots to share with you though!
We'll start with Snapshot 18w16a.
This snapshot added a brand new world type, called Buffet. Buffet worlds generate only a single biome, so this is a good world option if you want to try to survive in just one type of biome. There are two ways to generate these worlds. There is the normal overworld type, which spawns the biomes as they do in normal worlds (except of course they go on for infinity). There is also an end type, which generates the biome in a similar fashion to how the end islands beyond the main island in the end are generated.
Coral fans will now spawn naturally in coral reefs on the sides of coral blocks.
Blue ice is a little less slippery.
Coral blocks now require silk touch to drop itself. Otherwise it will drop as dead coral. Coral blocks will also no longer instantly die on land.
Drowned can now spawn in underwater ruins.
When water spreads and would later turn into a source block, it now immediately just places a source block.
Conduit Power particles (those little swirly things you see when you're under a potion effect) are now blue instead of orange.
Water breathing given from turtle shells no longer give off particles.
This next part is copied from Minecraft Wiki.
Optimized world spawn position–finding code.
New valid_spawn block tag for blocks that the player can spawn on.
There is now a distinction between scheduled "liquid ticks" and "block ticks".
New block and item tags for coral.
Changed how the temperature of ocean biomes is calculated; should result in nicer transitions between oceans.
Biome names are now translatable.
Updated some biome names:
Name ID Old name New name
minecraft:desert_hills DesertHills Desert Hills
minecraft:forest_hills ForestHills Forest Hills
minecraft:frozen_ocean FrozenOcean Frozen Ocean
minecraft:frozen_river FrozenRiver Frozen River
minecraft:hell Hell Nether
minecraft:jungle_edge JungleEdge Jungle Edge
minecraft:jungle_hills JungleHills Jungle Hills
minecraft:mesa_rock Mesa Plateau F Mesa Forest Plateau
minecraft:mushroom_island MushroomIsland Mushroom Island
minecraft:mushroom_island_shore MushroomIslandShore Mushroom Island Shore
minecraft:mutated_birch_forest Birch Forest M Mutated Birch Forest
minecraft:mutated_birch_forest_hills Birch Forest Hills M Mutated Birch Forest Hills
minecraft:mutated_desert Desert M Mutated Desert
minecraft:mutated_extreme_hills Extreme Hills M Mutated Extreme Hills
minecraft:mutated_extreme_hills_with_trees Extreme Hills+ M Mutated Extreme Hills+
minecraft:mutated_jungle Jungle M Mutated Jungle
minecraft:mutated_jungle_edge JungleEdge M Mutated Jungle Edge
minecraft:mutated_mesa_clear_rock Mesa Plateau M Mutated Mesa Plateau
minecraft:mutated_mesa_rock Mesa Plateau F M Mutated Mesa Forest Plateau
minecraft:mutated_redwood_taiga_hills Redwood Taiga Hills M Mutated Redwood Taiga Hills
minecraft:mutated_roofed_forest Roofed Forest M Mutated Roofed Forest
minecraft:mutated_savanna Savanna M Mutated Savanna
minecraft:mutated_savanna_rock Savanna Plateau M Mutated Savanna Plateau
minecraft:mutated_swampland Swampland M Mutated Swampland
minecraft:mutated_taiga Taiga M Mutated Taiga
minecraft:mutated_taiga_cold Cold Taiga M Mutated Cold Taiga
minecraft:taiga_hills TaigaHills Taiga Hills
So in the list "minecraft (biome name)" that is shown first is the name ID. Next the old biome name is shown, and lastly the new biome name is shown. We unfortunately have to learn some new biome names.
Although thankfully some of them are the exact same as before but they just added a space in between the two words.
Next up, snapshot 18w19a.
A new effect has been added! It is called Dolphin's Grace. This effect is obtained by swimming near dolphins and it allows the player to swim faster in water. How much faster? I'm not really sure, I'll have to test it.
Some stuff with fluids that I don't really understand. However in this snapshot it cannot be interacted with directly.
A new buffet generation type was added, caves. It creates nether generation. I haven't played around with this yet so I'll have to see how this works.
A couple of blocks got new names. Sea Grass is now called Seagrass. Tall Sea Grass is now called Tall Seagrass. Their ID names were also changed accordingly. It's a weird change I think. Speaking of Sea Grass Sea Grass will now generate in underwater caves.
The texture for the turtle egg in your inventory was changed.
The phantom spawn egg texture has been updated to better reflect the actual phantom's new textures.
The heart of the sea can now generate in buried treasure chests (in stacks of 1 and it looks like it is guaranteed to appear).
Clownfish has now been renamed to Tropical Fish. It feels kinda sad that clownfish don't exist anymore, though I think its texture is still being used. Also Puffer Fish Bucket has now been renamed to Pufferfish Bucket. Lastly the Pufferfish itself has gotten renamed from Puffer Fish to Pufferfish. It's ID name has also been changed accordingly.
All undead mobs, excluding zombie horses, will now sink underwater. It's unknown if the wither will do this as well since it normally flies. Also skeleton horses are now rideable underwater, which I think is pretty neat! It also gives another reason to own skeleton horses.
Phantoms will now flap their wings smoother and faster, although for some reason it won't work in some snapshots. Phantoms will also now drop experience.
Parrots will now imitate phantoms and drowned.
If dolphins start drowning (as they can drown and suffocate, so they need a mixture of both water and air), then they will swim to the surface.
Some world generation stuff has been updated, and thus worlds in older snapshots will not work in this snapshot (although this got fixed in future snapshots).
The output from the /seed command can now be copied.
Now for some general stuff:
Improved the upgrade paths from old versions.
The credits list has been updated.
Experience orbs will now float in water instead of sinking in water (which I find to be a major convenience).
The "How Did We Get Here?" Advancement now includes the Conduit Power status effect as part of its list.
Some new sounds for conduits and beacons have been added.
The F3 menu now shows the biome ID rather than the name.
Lastly several biomes have had their names changed, some more than others. However the list is fairly long so I'll just give you the link to Minecraft Wiki if you want to check out all of the new biome names. https://minecraft.gamepedia.com/18w19a
I'll put the rest of the snapshots on the next post.