And try to make tke bottom pointy

You can use floating blocks to make your tank float

Try putting vertical stabilizers in the back of the tank, this will stop it from sliding on the water. Here is a good example.

Keep in mind:
Have weight on the center-middle-low section of the vehicle. So like on the bottom plate. Have floatations roughly mid height. For steering, I suggest having the nozzles above a rotator. The rotator will act as forward/reverse, and set the nozzle's input to yaw or roll. Keep the VTOL engine as high as possible, otherwise it will be swamped.