SpoutDev/Vanilla — comment on commit 172d2337a37b20136899dc3d38c1f8b416bfe70e
File:  line 0
By spacechase0:

I think they're separate. They are mentioned in different paragraphs on the [MC wiki](http://www.minecraftwiki.net/wiki/Ice).

> Ice can be easily destroyed without tools, but using a pickaxe greatly helps. If there is another block directly underneath the ice block, it will revert back to water when broken. If not, it will shatter without producing water.

I remember that the water still melts without a block under it, though, because soon after I got Minecraft (before biomes), I had a snowy world and made my house under a frozen lake. One of the torches melted a single block of ice. :P