One way to deal with the meniscus problem is by undercutting the bank right at the desired waterline, pouring the resin to a depth just below the cut-back bank, allow to harden, then carefully add a final layer of groundwork to cover this interface. Depending on scene type, bushes, grass, tree limbs, etc., can be used to hide the meniscus. This technique can be found on page 82 of Osprey's; Terrain Modelling, by Richard Windrow. Good luck!
The other way listed in the above book involves a hair drier / heat gun and risks setting your diorama on fire...and your house.
We'll just skip that one for now.