I have a 2006 Maristar 200ss (same as X2) with a Crusader engine at 1100+ hours. Black dust, in my situation, has been normal. I always-always-always carry a spare belt because they will deteriorate & break when you don't want them too. Boats don't go without a serpentine belt.
Also, the "belt track" on the water pump pulley has also been normal and needing replacement every blue moon. Why does this happen? It's a cheap part and painted. My problem is that I primarily run in Saltwater so rust gets on the pulley and helps kill the belt more rapidly. I finally got smart and started inspecting the belt on a regular basis for wear and tear. Noticeable engine performance with a new belt vs a belt that has decayed a bit.
The water pump pulley is only ~$40 and takes about 5 minutes to change out. Remove the water pump pulley assembly from the water pump. The pulley is on there snug, so I use a socket and gently tap the pump house to push it off the pulley. The new pulley may not seem to fit because of the paint thickness. I just start the bolt threads and replace the assembly back on the water pump and tighten it down, slowly and evenly.
Also, it sounds like your Dealer is just making conversation and has no clue. The dealer where I bought my boat was an idiot and it took me about three mechanics to find one that was honest enough to say, "I don't know, let's monitor it". Been going to that mechanic for about 800 hours. Good luck!!!