The main differences are:
The Rocker: boat boards are more likely to have a 3-stage rocker for more pop off the wake. Cable boards lean towards a more continuous rocker for smoothness on the cable.
The Flex: Boat boards tend to be stiffer as they need that tension from the board to pop off the wake and stay really solid in the landings. Cable boards tend to much softer for the flexibility to hit rails, the softer boards can 'grip' or 'hug' the rails better.
The Base & Fins: Boat boards have a more defined channel system on the base, including moulded fins and also removable fins. These help with hold edges while cutting into the wake. Cable boards are 'naked' on the base, they have evolved and steered away from moulded fins and removable fins over the past few years. The clean bases allow for a smooth ride over park features.