you can use one esc for both motors but it should be a high amperage esc at least 40A but i would go higher around 80 personally, the mixer is not really needed but you plan on using a 2 channel TX so its really a must since you will have to edit the esc cables to both go into 1 port in the receiver, the mixer does all the jobs in one why i mentioned it.
you can if you wish ... controll each motor separately using a 4+ channel transmitter each motor is on a separate stick but it takes a load of practise to get it to go straight and you have to balance the power of each motor with the turn of the rudder, but you do have the option to not use a rudder with that type of setup but i would ~NOT~ recommend that until you are More confident in your ability's to control a craft on water.. its not like cars can stop on a dime or turn hard and not flip.
the mixer just makes twin motors easy i have never made a boat with twin motors and not used a mixer of some type, even when i built a tank a mixer was definably needed there!
give it some though even ask others about mixers using 2 channel radio gear see what they say , but to me spending a further £18 on a mixer is well worth the hustle of steering, Remember there might be others on the lake/pond that wont want something crashing into them at high speed because you haven't got full control of it , as well as animals hitting one of them is seriously EXPENCIVE!