What is your budget?
You boat is a pretty much perfect size for a 4s setup with an 18-2000 kv motor, An aquacraft UL-1 or Revolt motor would work great with an inexpensive turnigy 120 or swordfish 120 esc.
Also gives you a boat that fits right into P (4s) limited or spec mono, class usually runs a pair of 2s5000mah batteries in series. Has to run a motor available in either a proboat or aquacraft rtr boat, open esc.
There are some cheaper options for motors but you would have to run against open class motors where guys are spending huge money on motors and batteries.
Doesn't matter so much if racing is not in the books.
Going down in voltage with the same size motor and prop will increase the amp draw and the heat, not really what you want.
Going up in voltage means a lower amp draw all things still equal, better option.