海 and 湖: locations around seas and lakes
Distinguish water bodies from viewing directions and visit locations.
Going to the sea
海 (うみ) means sea. 「夏に海へ行きます」 plans a visit to the sea in summer.
「海の近くに泊まりました」 means staying near the sea, not physically inside its water.
A lakeside scene
湖 (みずうみ) means lake. 「湖の岸で休みました」 means resting on the lakeshore.
「湖に山が映っています」 says a mountain is reflected in the lake. The mountain itself has not moved into the water.
Visibility versus actual location
「部屋から海が見えます」 says the sea is visible from the room. It does not establish that the room is immediately beside the shore.
「海ではなく、湖で泳ぎました」 places the swimming in a lake, not the sea. Properties such as salinity are not stated.
Does 「湖に山が映っています」 put the mountain inside the lake?
Show the answer and why
No. It describes the mountain's reflection on the water.