“Submersion” is episode eighteen of season three of Stargate: Atlantis.
As was found out in the season one episode “The Gift”, Teyla possesses some wraith DNA, allowing her to sense the Wraith. This has the unfortunate side effect of sometimes allowing Wraith to control her.
The Atlantis team is exploring the oceans of the planet again in another Jumper, looking for an underwater Atlantean mobile drilling station to help with their power needs.
Their approach awakens a Wraith queen in a Wraith cruiser and they wind up trapped aboard the station with a very powerful Wraith – who has set her cruiser to self destruct, which will cause massive devastation.
A pretty standard race against time with a literal deadline to beat. Being trapped with no way out adds a certain degree of pressure to the situation.