Just for reference, this is the first RC Car that I got for my 4 year old Grandson. It works in water and on land.
Kelaton RC Cars 360 ° Rotation Four-Wheel Drive Water Spray Amphibious Car Water Cannon

The throttle is on/off, and steering is by left and right buttons. Neither is proportional, but it has been a lot of fun. You can run it in the pool or on land. It has a water cannon, which is fun. It is clearly a toy class, but still enjoyable.

It is fast enough on solid ground to be fun, but not so fast that he can't control it. It doesn't have enough ground clearance to do well on anything but the shortest grass.