QIKsilver:
Adding to the above suggestion of clear sheet to support.
I have seen a similar topic to what you're asking about. The outcome depends on your skill level, however it is still fairly easy.
1. Form a sealed box the size of the finished diorama.
2. Layout the scene and size the water area
3. Block in the water area with plasticene and place the AFV or vehicle in the water
4. Sculpt the "displaced water" in waves from the plasticene and brace the curves and angles solidly
5. Remove the AFV or Vehicle (make sure the waves are where you want in height etc.)
6. Mix clear resin with the chosen colour and pour in thin layers on the top of your plasticene molds.
7. Continue pouring layers until the desired result is achieved
8. When the resin is finally set remove the plasticene mold support and finish paint and any touch-up required.
9. When you're finished you should result in formed waves and large splashes that can be anchored in to the base and a thin layer of resin will tie the AFV or Vehicle in to the river and blend the splashes to one uniform looking body of disturbed water.
10. Final stage is to add a few gloss coats and maybe some microballoons thrown in to simulate bubbles.
I hope this helps you achieve what you want.
Cheers;
Gregory