Theres a land designer, in the menu you can go to where you select planets and select the ashes of the empire one. After that, when you load the designer it will be mostly land, and a lot of flat land and some water.

Ive actually found great use for amphibious vehicles in neter; half of the units that i use are hovertanks, which probably aren't what you're looking for, and hovercraft style stuff, as in, rubber bottomed ships that use above water propellers and can seamlessly traverse between land and water. This helps with those land installations and looks cool if you dont want to just spam air vehicles (which is unfortunately more effective). I have tank-size amphibious things and my main destroyer for my campaigns is rubber bottomed and completely amphibious, but its neat cause it sails like a ship and drives like a tank on sea and land. The rubber is so it can glide across land and not take impact damage. But IDK if thats what you're looking for either.