Has to be coated on all sides, When most people "coat things" in water, they forget to do the bottom.
Hornom, I'm going to test that later but I don't think that works because people can easily glitch into blocks. The way I protect my chests is to have them not touch any block but water, so water above, water must be on all sides and on bottom, unless you want to stack them, then water would be on top of the highest one and the chests can be stacked. Hope this helps, message me if you are confused, I'll try my best to help. I know I'm not very good at explaining, but I'll be happy to show you on creative.
I haven't tested it, but that is probably the case. People can place creepers fully in chests though and just glitch into them. Normally the creepers wouldn't blow up because they can't see anyone, but if someone glitches in, the creeper can see them because they are in the same block. I don't know if this is the case with suffocating blocks though.