Two ESCs. The greatest benefit of two ESCs is that you get to control the motors independently, much as the real thing. Juggling the sticks syncs them. That, or using a mixer in the boat or a mixing facility if the transmitter has it. They can be set up on a Y lead so that both plug into the same channel. Assuming that they can be set up (some autoset, some are as supplied), if they are set up matching, they will sync. More or less.
One ESC. As long as the one ESC can handle both motors under load, it will generally work fine as long as you are aware that the motors themselves, and the loads on them, will vary within some sort of tolerance. If using two motors off one ESC, it is a good idea to fit a fuse for each motor, if anything 'orrible happens to one motor, hopefully the fuse will protect the ESC and leave some power to get the boat back.