In order to get sea missions you need to set I spawn a mission spawn area. To avoid ocean missions taking place on land it is important to set the spawn are at sea.
In order to set a spawn area, go into the station scene of your coastal rescue station and click on the „Set mission spawn area“ button on the bottom of the page. You will then be able to define the radius and to move the center point of the mission spawn area (the area is always circular). Sea missions generated by that coastal rescue stations can only occur inside that area, so make sure that the spawn area only cover sea area and doesn’t spread to the land.