Welcome to the forum. So, it sounds like you're trying to build a paludarium, a vivarium with land and water. I've got the same kind of project I started a while back with a 29g. There's actually a sub-forum that deals with these. I'd highly suggest going there and doing some reading. Under the General Planted Tank Discussion there is a sub-forum for Riparium/Terrarium/Vivarium. Do some searches for building paludariums. There's a lot of threads, and it turns out there are a lot of wrong ways to do it. A little reading there can save you a lot of heartache down the line.
You can use silicone to attach rocks and such but they may not make a proper barrier between land and water.
Most people build rocks and structure with rigid foam covered with spray on expanding foam "Great Stuff" and then covered with tinted cement products.

 

I have built these twice before. This type of project will definitely test your patience the first time, I agree with Kubla research and forums will help a lot. The Dendra Board is a good forum for this as well. For me I found that products for ponds works very well, but using a cut piece of acrylic as a barrier works best then I used the Great Stuff for ponds that is black to shape rocks covered with pond cement (or any animal safe cement that won't harm the animals) to give it the natural texture. This all keeps the weight down and the flexability to shape and mold it to what you are going for.