row a boat, paddle a boat, finger guess
划 centers on propelling a boat through water with an oar or paddle. The "finger guess" meaning is a rare, unrelated sense that does not connect to the rowing idea.
划 combines 戈 (a halberd-like weapon) with 刂 (knife), but the historical connection to rowing is unclear. The modern form may be a simplification or borrowing, and the exact origin is uncertain.
Picture the left side 戈 as a long oar and the right side 刂 as the blade cutting through water. Together they show rowing a boat.
For カ, imagine a car floating on water and you rowing it with a paddle: car -> カ, and you paddle the car-boat along.