How much A ESC is appropriate for a 90cm race boat model?

Diamond Dynamics 90A ESCs are recommended.

There are five types of Diamond Dynamics model boat ESCs: 50A, 70A, 90A, 120A and 150A, corresponding to the recommended maximum boat length: 55cm, 70cm, 100cm, 120cm and 135cm.

The Diamond Dynamics model boat series ESCs are equipped with a high performance MCU processor and a built-in 5.5V/5A SBEC for fast throttle response and very smooth speed linearity for all kinds of model boats.

The ESCs can be programmed by card or remote control to set unidirectional or bidirectional operation mode, motor forward and reverse, low voltage protection threshold, starting acceleration, feed angle.