For the lighthouse, the entrance is not by sea.. It is in the southeastern part of Olivine City. From the Pokemon Center in Olivine go east (to the right) untill you hit a tall building. Then go down and to the right and thats the entrance to the lighthouse.

And to surf to Cianwood you are probably entering the water at the wrong area. From the Olivine Pokemon Centre go west (left) and enter the water by ...

For the lighthouse, the entrance is not by sea.. It is in the southeastern part of Olivine City. From the Pokemon Center in Olivine go east (to the right) untill you hit a tall building. Then go down and to the right and thats the entrance to the lighthouse.

And to surf to Cianwood you are probably entering the water at the wrong area. From the Olivine Pokemon Centre go west (left) and enter the water by the route 40 sign. Go south and west (down and the the left) and you will reach Cianwood. You will get the medicine in a house on the south of the city, and after defeating the gym, you can obtain the HM for fly outside the gym and fly back to Olivine.