The fuse value will depend upon the type of motors used and the type of speed controller. It is better to measure the current consumed under load i.e. (boat in the water full speed and tethered to side of pond). Assuming the total current was 8 amps and the speed controllers were rated at 10amps each, 15 amp would be a reasonable value for the fuse.

Depending upon the capabilities of the modeller, it is good practice to measure also the stall current, when one motor is stopped from turning at full speed. The current consumption will rise dramatically and this should blow the fuse. If not lower the value of the fuse until it does.

In the example shown in the diagram both motors are driven independantly. It is possible to connect both motors in parrallel across one suitable capacity speed controller, if you do not want independant control. Again this depends upon the motors used. Trying the motors on the bench under heavy load conditions (motor shaft slowed down by holding with a rag) at full voltage and measure the current, will give you a good idea what current capacity the speed controller needs to be. 

                             DON'T FORGET THE GOLDEN RULE

                                   Don't connect Motors in Series