I have a 3 acre pond, and plan to treat 1/3 at a time at 2 gallons per acre.
So, I am thinking of adding 2 gallons of reward and 13 gallons of water to my 15 gallon sprayer. Use my boat and keep covering the 1/3 acre until I use it all.
Assuming your pump's flow-rate is 1gpm (for example), and using your referenced mix-ratio, you would have 15 minutes to treat 1/3rd (1-acre) of your 3-acre pond at a use-rate of 2 gpa (of REWARD).
If you can effectively cover the designated area in that time-frame, your plan is fine.
If, instead, you think it will take 30-min to cover a 3rd (1-acre) of the pond, then mix 1-gal of REWARD with 14 gallons of water - and disperse two of these tank-loads over a 3rd (1-acre) of the pond.
In both scenarios, you've applied 2-gal of product per treated surface-acre. The ratio of mix-tank water-dilution is irrelevant.
Adjust the mix-ration based on your pump's flow-rate and the amount of time required to travel/cover the designated area.