Check the blocks that are over or under the target. Ur example is to check the depth of the water where a bobber lands, but I'm sure creative souls could come up with other uses. (Is your target standing under glowstone? Chance to drop dust)

Can you elaborate more on this? I'm not sure what you want. It doesn't sound like you want to just check the blocks directly over or under the target.

I believe you are correct. I'd like to be able to check if the (x-y# of blocks) under the bobber is water, for instance. I'd also like to be able to check if the (x-y# of blocks) over the target contains or does not contain something, and potentially both on the same target.

A set of examples:
1 - Are the three blocks directly under the target water?
2 - Do the three blocks above the target contain at least one instance of glowstone?
3 - Are the three blocks directly under the target water AND do the three blocks directly above the target contain at least one instance of glowstone?
4 - Are the three blocks over the target glowstone (Must have three blocks of glowstone over the target)?
5 - Do the three blocks under the target NOT contain sand? (An inverse of the above logic)
6 - Is the first non-water/air block under the target sand?
7 - Is the first non-water/air block under the target NOT sand?
8 - The above two examples, only checking up instead of down.

Preferably multiple conditions should be able to be applied to a single target.