Envirotex Water Thickness - Model Railroader Magazine

If it doesn't say in the instructions, I would pour up to 1/2" at a time, and no more.  If that hardens nicely and tacky free in about 6-10 hours, I would then do another pour if you desire a lot of depth.

Note that 1/2" in HO scale is really 44" in depth.   Not too many natural accumulations of water allow you to see far into it.  If you do two such pours, now you have 87" of water, and that gets quite dark, especially if there is any vegetation and rotting matter down at the bottom.

Do you have to have it that deep?  And BTW, are you aware that you will have to fully seal the edges where the foam and wood surface meets?  You will have to use a caulk or a putty of some kind.