Can anyone tell me how to make a splash for a vehicle entering a body of water? What material would I use to make the splash and what is the best technique for making it?

Resin is it. I've seen some really nice splashes. It takes time, the ones I've seen were built up slowly and formed as the resin hardened. You can also make a mold, grease the mold do it can be removed then pour into the mold.

Check out this link
https://armorama.kitmaker.net/forums/6264&page=1

Thanks Scott.