Principally in this case the Mediterranean as this is the sea of Cyprus but not exclusively.
Interestingly we also find thallasa used in a non-marine context specifically in the phrase egana to thallasa or literally "I made it a sea".
This means that the speaker made a total and catastrophic hash of whatever they were referring to.
You will probably hear more frequently other derivatives of the phrase such as eganes to thallasa -"You made it a sea", or else egane to thallasa – "He/she made it a sea."
This is because the likelihood of a Cypriot admitting that they made a mistake varies from slight to nil.