In the midst of replacing my toilet and trying to lay down new vinyl, we have found that the wood flooring underneath the vinyl is wet, black and rotten. We thought it might be because the vinyl was not caulked correctly by the tub, but now I'm thinking that maybe the drain or the pipes for the tub faucet and shower head might be leaking. I have nightmarish images of having to tear down the tub and the surrounding tile to be able to diagnose the wetness. Am I close, or will it not be that hard to diagnose? If it's that hard, I need to save up some money before I call a plumber over. We can do the sub-flooring and tub-replacement ourselves, but we're lost when it comes to the plumbing stuff.

I can look into my bathtub drain and see standing water right near the top.....is this normal also that same pipe is leaking at the trap......any answers on what to do?