====== Make a Block Waterloggable ======
To make blocks waterloggable, implement ''Waterloggable'' interface and override some methods.

<code java>
public class VerticalSlabBlock extends HorizontalFacingBlock implements Waterloggable {
    [...]
}
</code>

In this case, we store the ''Properties.WATERLOGGED'' as a static field in this class, and you can use ''WATERLOGGED'' to access the field. (Unnecessary as it looks, most vanilla Minecraft block classes do that, so we do as well. Of course you can refuse to do that, and use ''Properties.WATERLOGGED'' every time to access it, or just directly import the static field.) We have to manually make the block recognize the property, as it is not done for you by ''Waterloggable'' interface.

<code java>
    // Note the ''Properties'' is net.minecraft.state.property.Properties. Don't import the wrong one.
    public static final BooleanProperty WATERLOGGED = Properties.WATERLOGGED;

    // Let default value of the WATERLOGGED property become ``false``
    public VerticalSlabBlock(Settings settings) {
        super(settings);
        setDefaultState(getDefaultState()
            .with(Properties.HORIZONTAL_FACING, Direction.NORTH)
            .with(WATERLOGGED, false));
    }
    
    // Make the block recognize the property, otherwise setting the property will through exceptions.
    @Override
    protected void appendProperties(StateManager.Builder<Block, BlockState> builder) {
        builder.add(Properties.HORIZONTAL_FACING, WATERLOGGED);
    }
</code>

We also have to override ''getPlacementState'', so that the block placed in water is initially waterlogged.
<code java>
    @Override
    public BlockState getPlacementState(ItemPlacementContext ctx) {
        return this.getDefaultState()
            .with(Properties.HORIZONTAL_FACING, ctx.getHorizontalPlayerFacing().getOpposite())
            .with(WATERLOGGED, ctx.getWorld().getFluidState(ctx.getBlockPos()).isOf(Fluids.WATER));
    }
</code>

Override ''getFluidState'' so that when it is waterlogged the block displays water.
<code java>
    @Override
    public FluidState getFluidState(BlockState state) {
        return state.get(WATERLOGGED) ? Fluids.WATER.getStill(false) : super.getFluidState(state);
    }
</code>

Override ''getStateForNeighborUpdate'' so that it correctly handles the flowing of water.

//For versions 1.21.1 and below, write like this://
<yarncode java>
    @Override
    public BlockState getStateForNeighborUpdate(BlockState state, Direction direction, BlockState neighborState, WorldAccess world, BlockPos pos, BlockPos neighborPos) {
        if (state.get(WATERLOGGED)) {
            // For 1.17 and below: 
            // world.getFluidTickScheduler().schedule(pos, Fluids.WATER, Fluids.WATER.getTickRate(world));
            // For versions since 1.18 below 1.21.2:
            world.method_39281(pos, Fluids.WATER, Fluids.WATER.getTickRate(world));
        }

        return super.getStateForNeighborUpdate(state, direction, neighborState, world, pos, neighborPos);
    }
</yarncode>

//For versions 1.21.2 and above, write like this://
<yarncode java>
    @Override
    public BlockState getStateForNeighborUpdate(BlockState state, WorldView world, ScheduledTickView tickView, BlockPos pos, Direction direction, BlockPos neighborPos, BlockState neighborState, Random random) {
        if (state.get(WATERLOGGED)) {
            // For versions since 1.21.2:
            tickView.scheduleFluidTick(pos, Fluids.WATER, Fluids.WATER.getTickRate(world));
        }

        return super.getStateForNeighborUpdate(state, world, tickView, pos, direction, neighborPos, neighborState, random);
    }
</yarncode>

Now the block becomes waterloggable, and works correctly with water.