It’s highly likely the whole floor is wet. But there is no harm in cutting it out and inspecting. Any sign of wet beyond your cut, keep going. It’s probably easier to replace a whole flat panel than patch a piece in.
If you intend to patch then cut it on an angle 30-45deg. Then cut your replacement on angle too. Then your new bit won’t fall through the hole you cut.
Unless it’s a massive floor I’d chop it all out. If one bit is wet the rest might be. Including knees or anything connected.
You gonna use polyester?