I have been in 3 times now and have to say I have not noticed this.I received one "off" pint of ghost ship but just returned it and got another which was fine.
For anyone interested, altogether cask wise I have had Ghost Ship, Broadside, Dark Arts and Bru Ri. All were tasting as I would expect them to taste and not watery in any way.
Dark Arts was was great, only time I've ever had it on cask. Shame it was gone in about 2 minutes though!