what you should do is get some water fall expanding foam, and build the "wall" that will be between the water and land portions.
be sure to clean your glass very well with alcohol and then rinse, and dry good. use a marker and draw your idea or like i've done is draw it on cardboard then set that under the tank.
get some aluminum expanded metal; in the chicken wire area of home depot, and with leather gloves, build up your shape. place
it in the tank along your pattern. shake foam up, and with can upside down, poke nozzle into the wire mesh form. once your fill the inside of your "wall" let the foam dry. be creative but real.
with FBTs watch the accessability of your wall|divider.
once it's dry, cover with vivarium mortar or portland cement, the later is cheaper! you only need to mix a little at a time, and mix a stiff mix, 1:1 with distilled water. using rubber gloves, hand apply this to the dried foam form with a thickness of 1/2" or so. use sand to add texture and even some gravel towards the bottom for a more real effect. i've used a 1:3 (Cement:water) mix and coated parts after the main part was dry then added the sand and such. even coated leaves once and that worked great...Off TOpic...
once the cement is dry (a week), using a non toxic sealer (my site) as it's able to seal the cement so there is no change to the pH and no leaching at all!. apply this with hand held spray bottle though as a brush won't make the application level. really soak the whole thing.
once that is dry, which depending on the thickness and your application could take a day or two, you can now add your soil and water.
email for photo links if you want. i tried here but got the red X issue.
hope this helped. what size tank is it? also, what kind of filter are you using? if you want a water feature on the land or just want to run the inlet hose to the other end of the tank from the filter side, just lay the hose first then foam, etc.
pretty easy though.
Sincerely,
Deven Nicholson
Terra5Designs | Terrariums and Naturalistic Vivariums
925-465-1295
http://www.Terra5Designs.com
Ensuring Animal Husbandry through Education and Habitat Design.