I guess I just don't see the stain ... the wood appears to have a very nice pattern and coloration. Sorry, just not seeing a stain. Is it really that obvious? I agree, that if it looks like a stain or otherwise, it should be looked at, but I wouldn't be alarmist based on a grainy photo of wood (teak - possibly varnished or oiled) that has a lot of dark coloration. Wood has an amazing capacity for tolerating wetness periodically without any structural degradation. Even stains can be apparent without any damage. Teak, in particular, is extremely resistant to rot. However, as you know, the teak veneer is backed by a softwood. It is an easy task to suss the condition of exposed wood. The fact that it is exposed will allow the wood to dry easily, making it far less likely to deteriorate. Concealed wood in damp areas is far more problematic and I admit that a stain leading to a concealed location is suspicious. Admittedly, the bottom edge of the plywood is susceptible to damage. My bulkheads suffered damage around the edges that required reinforcement.I don't think he was being alarmist. If I was a novice and had that stain on the bulkhead below the chainplate I would appreciate someone having pointed out that it was a possible serious issue and to check it out.
I still think the photo doesn't clearly show a problem. Whatsit hasn't suggested that he has seen any damage, just water on the floor, which probably comes from the wire penetration thru the roof under the mast, as has already been suggested to him.