I believe negative layers make it a (sky) background, thus indeed appear behind anything else. Start at 1000 or something if you need to go lower.
The manual says:
A7.06 Panels, texts, and view entities with a positive layer value are placed above any view; with a negative layer value they are placed below any view. This can be used to define an image visible in the background of a view that has no sky and a transparent sky_color.
To detect you're in water or not, read about pixel_for_bmap in the manual. Check the pixel that is in front of the tank. If its value is blue, you know it's water.
