Hi Ribbity...I noticed no one has answered so I'll chime in with my teeny tiny bit of knowledge.

You can purchase silicone in the plumbing department at the hardware store (home improvement store? what do you guys call it in the UK?) There is a brand called DAP that is clearly labeled "aquarium safe" or "animal safe." Or you can go to the Aquarium section of the pet store and look for aquarium silicone. Beyond that I don't know what types people use.

Also, I'm not clear on what you're trying to divide. Are you doing a simple land/water divisiion? Do you want the land area completely water-proof and sealed off from the water portion? That may require a glue AND a silicone, but I'm not certain. Or do you want the water from the water section to run underneath the land section as well?

From what I can gather, there are a few different ways to do it. Your land portion could be completely elevated from the water portion, with a slope that the frogs can wander down, like a frog beach Or you could have it all on the same level, but divided.

I only know this because I've been researching it a bit lately. Haven't done it yet. Hopefully soon someone will have more info.

Good luck!