River table?

I've only used Amazing Clear Cast epoxy as a topcoat, I don't know how it behaves in thick pours. Here's their page on it, scroll down to find data sheets and usage instructions.
www.alumilite.com/resins/amazing-clear-cast/
They don't say anything about pour thickness, just topcoat thickness.

Personally I would lean towards the urethane products (clear, clear slow, water clear) which I regularly pour at 4 and 6 inches thick in one go. The urethanes are prone to bubbles though; I use a pressure pot which you obviously can't use so you will need to degass the resin in a vacuum chamber prior to pouring.

Another consideration is the final polish the products will achieve.
*Disclaimer: the following opinions are derived soley from my personal experience with several manufacturers epoxies and urethanes. Your mileage may vary.*
The urethanes will polish to a glass like finish. The epoxies polish well but always remain somewhat hazy on the surface. The epoxies will cure perfectly clear, but if you need to sand the surface they will not polish back to that optically clear state. Urethanes will. If you are using opaque colors the epoxies work just fine. If you are wanting transparent ot translucent colors I think urethane is a better choice.