If the name of the ship were known it would be your best option.
But once we're into the sub-optimal: given the fact that the banner you linked to reads, "9th Conference on Sustainable Development of Energy, Water and Environment Systems, September 20-27, 2014, Venice-Istanbul", I think you are on safe ground to record the ports of call as locations associated with the conference, per ES 18.104.22.168, which would likely result in an access point that looked something like this:
Conference on Sustainable Development of Energy, Water and Environmental Systems (9th : 2014 : Venice, Italy and Istanbul, Turkey)
Doubtless you are correct the conference actually took place on the line described by these two points, but I think it's accurate enough that a researcher looking to identify the conference would understand what is meant.
And while I'm not sure you can use a non-jurisdictional term (it certainly strikes me as novel) in this case "Mediterranean Sea" would be less specific.
(Of course, to further complicate matters, it seems the conference actually began at a third location, Dubrovnik. And indeed it could also be recorded as a place associated with the conference... but once you get around to using three locations in an access point things can get kind of messy: "Dubrovnik, Croatia, Venice, Italy and Istanbul, Turkey.")
From: Program for Cooperative Cataloging [mailto:[log in to unmask]] On Behalf Of Finnerty, Ryan
Sent: Tuesday, October 04, 2016 9:01 PM
To: [log in to unmask]
Subject: [PCCLIST] Conference held at sea
We are trying to make an authority record for the 9th instance of this conference:
The conference was held at sea on a ship going from Venice to Istanbul.
What would you put for the location of the conference?
I'm leaning towards using Mediterranean Sea but I don't know if that would be allowable. All the examples for conference locations in RDA are jurisdictions. I don't want to use Venice and Istanbul since it wasn't actually held in those places.
We don't have the name of the ship, but if we did, could you use the name of the ship for an "associated institution"?
Or would you just omit this element altogether?
Thanks for your help,
Ryan J. Finnerty
Head, Database and Authorities Management | NACO Coordinator UC San Diego Library | Metadata Services [log in to unmask]<mailto:[log in to unmask]> | (858) 822-3138