SANTANDER FERRY AND DAYS IN SCHENGEN

Hi Everybody ;-)
First off a big thanks to Gwynj for answering my last post so promptly and thoroughly.

However, that brings me onto another question wrt the UK - Santander ferry.

The 90/180 day rule is based on when a person enters/exits the Schengen Zone.
In which case, one would assume [dangerous ;-I] that wrt to, say, the Portsmouth to Santander ferry it applies to passing Spanish Customs on arrival at Santander.

However, the ferry is French owned [Brittany Ferries] and could therefore be considered part of the Schengen zone for legal reasons. 
Further to embark, one would have to clear EU customs at the UK Port, effectively leaving UK and entering Schengen
In which case it "could" be argued that the clock starts ticking on embarkation in Portsmouth a full 24hrs [1 day] prior to physical entry into Spain and therefore affecting the calculation of a persons 90 days entitlement.

And, wait, there's more ;-)
If the above does apply [i.e. the ay spent on the ferry counts as a day in Schengen] then surely the reverse applies and the return journey will also count as one of the 90 days.
And that could catch unsuspecting people out, by accidentally overstaying whilst still in transit or by omitting those 2 crucial days.

Hmmmmm?
Anybody any ideas or arguments for/against, information or facts to contribute?

I will be contacting Brittany Ferries direct and will post the outcome of that query.

Cheese Gromit

Max