I can't set material for river or lake. I tried drag and drop material from files list. Nothing happens.

To create a new water material you can right-click in the list on the left and choose
'new' -> 'water material'.

To use a water material for a new lake/river, open a game world and select the button 'Water'. Then you can drag/drop the water material you want to use when creating new water in the material slot that appeared when you clicked the 'Water' button.

To change the water material for an existing lake/river: select the lake/river and then drag and drop a water material in the box with parameters that appears for the selected water.

Thank you I tried use simple material.