The Raw Kelp Block

This block is brighter than the dried one, but from raw kelps.

It's a special block because this block has gravity, falls down until hits the ground, but in water... it floats upwards and as the block is half outside the water, it will jump and sink in this form-https://www.desmos.com/calculator/xy5qvdac3s 

However, unlike all falling blocks you can walk on this block and touch it, simple idea can be added