i think it would be neat if there would be a air bubble you could ware so you could breath longer under water could be made of glass and i spend most of my time in the game building my under water city and i find that it would be AWESOME if i could stay longer
Or really any block that you can walk 'through.'
Ladders
signs
levers
buttons?
doors
hatches
sugar cane?
feel free to expound on this list through rigorous testing. And stuff.