I've tried using dividers, but soon passed on them for much the same reason Jennifer mentioned, as the land area sound became boggy.
Instead, I decided to build up my land area taking cues from Ma' Nature.I created a mix that was of 4 sizes of sand and gravel, prettymuch evenly by volume, and to this added added about 1:2 ratios of peat to the mix. I wetted this down and stirred it well, till I could grab it out in clumps.
I covered the bottom of by tank in a layer of coarse .5-1 inch gravel. Then, on the "dry" end of the tank, I put down a layer of my mix, followed by a layer of large gravel and rocks, followed by another layer of mix, etc. Each time, I tapered a little further away from the "wet" end of the tank. I repeat this until the mount of material is about an inch or two above where I want my high waterline.
I tamp down the mixture with mild pressure in order to reduce any unexpected settling.
I then cover all al this is a 1:1 mix of peat and sand.
I keep everything very moist, and introduce moss and plants, which help to stablize the earthen mass you've built.
Now to fill it with water! I usually place a large rock, a little taller than my waterline on the wet end of the tank. I then SLOWLY pour the water in onto the rock. The rock diffuses the force of the water, so is doesn't tear up the lakebed I've built.
I've done this for myself and others, and, while it take a while to setup, looks good, is easy to maintain, and accommodates the critters nicely.