The Opal Sea was a sea on the planet Betazed.

In 2379, William T. Riker and Deanna Troi planned on having their honeymoon at the Opal Sea. Their trip was delayed when the Enterprise was diverted to Romulus, while en route to Betazed. (Star Trek Nemesis)

It is unclear whether Riker and Troi continued to the Opal Sea following the mission to Romulus, as Riker then became captain of the Titan and told Picard that the ship was returning to Romulan territory.
In Star Trek: Titan, the Opal Sea is not on Betazed but on a planet called Pelagia Prime.

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