I would have to disagree. I recall reading specifically a post from Pocus (probably back in AACW days) stating that artillery lead by an unactive leader or in PP will not bombard passing ships. It would break with the general PP paradigm that units in PP are 'passive', and bombarding is not passive.
I don't understand what putting the ships in OP should do, if they are already getting bombarded in DP.
IIRC it's very chancy to try to sneak past a fort. My conclusion was that it is better to load a fleet with ironclads, which will be the main targets of a bombardment, than to hope to sneak by with small fleets. The ironclads soak off the hits, and the rest get by practically unscathed.
At one point during beta testing for patches I noted that transports were taking an inordinate number of hits and being sunk, sometimes without a report of their loss, even if the entire unit was lost. We discovered that elements with generic names were not being reported. If the model files did not have enough names for all the elements of transports being built (names for each boat) those missing a name got a generic name like 'riverine transport', and their loss was not being reported.
This is why I spent many hours researching steamboat names of the era and why we have them now. I put a list of a couple hundred names together, and now they're not being reported again.