Following the sage Herb you can be
iver & ake
With six, I'm a few of your counterfeit pounds.
F= 6 → fake fiver
With twelve, in your body, and a body of water.
L = 12 → liver & lake
With eighteen, a body of water to clear the ground.
R = 18 → river & rake (e.g. leaves)
With twenty-two, taking a car swimming in a body of water.
DR (4 + 18)= 22 → driver (someone taking a car) & drake (male duck as may be found swimming in a body of water).
Thanks @J. Siebeneichler for Drake
With twenty-seven or thirty-eight, I'm probably terrified.
S (19) + H (8) = 27 → shiver & shake
Q (17) + U (21) = 38 → quiver & quake
With thirty-one, a small piece as a body of water helps you.
S (19) + L (12) = 31 → sliver (small piece) & slake (quench thirst)