Due to the map's simplicity and, presumably, small size, I suggest you max out on your level design knowledge. Add pipes to the walls, play with lightmap settings, do some dramatic lighting, add particle effects, sprites, make cables hang from the ceilings, add some more height variation and details to the walls and ceilings, such as trusses and supports.
I, for one, would keep the map concept and... general mood (dark, dank... dripping in the distance) and instead make it legitimate. In this, I mean make it an actual place. I suggest an underground parking area. This means you can add cars and all the things I mentioned previously, and players won't be like "What is this place?".
Currently (prospectively) you have a pool of water in the middle of some random room lit by perfectly white lights surrounded by cars in a largely bland surroundings. This map could be so much more if you turned it into a real place, with all the detail you can possibly muster.
Underground parking lots also mean there are high contrast areas that you can use to attract survivors, as well as make it generally more creepy. The above picture isn't a very good example, but you see what I mean.
No matter what you do, however, I suggest you change the lighting either to a pale green or a pale blue. These are essentially creepy shades, particularly green. Look up 'light psychology' possibly in the Valve dev wiki and it'll explain lighting shades.
I hope some of this is some kind of inspiration to you, and I hope you take heed of my advice
if you post reasonably regular updates and screenshots (with the changes I suggested) then I can give you straight criticism of your map, and direct advice... mainly concering lighting, as it's kinda my thing