Has it suddenly started this, or have you bought it like it, or what?
When you say BMW, is it the VM engine? (i think some people refer to it as a BMW sometimes)
If it's smoking (black) it clearly can't burn the fuel, so either not boosting (as you say) because the turbo is in trouble, or maybe it's (the turbo) unable to breath? exhaust wet bend/elbow, or lower pipework/drive exhaust route restricting it's exit in some way, maybe through corrosion or even an upstream restriction, i.e., blocked or collapsed air filter or induction pipe?
If it wasn't smoking, fingers would point toward the pump/injectors, but black smoke means insufficient oxygen for the amount of fuel, i.e., LOW BOOST.
Oooh, and lastly, does this boat live in the water on a mooring? If it does, when was it last lifted and barnacles etc removed?
"I Agree with everything you say really!" - John Cooke to Jon Fuller - 26-01-2013