Yes, V8 is a good choice. There are crime-prone areas in Mexico
(check noonsite.com for information) and when the wind
is dead, you are going to want to be able to get away in a hurry. Not sure about the fuel tank
size. Go large.
Dulcesuenos' suggestion of a paddlewheel is good. The markup on boat propellers is outrageous. Paddlewheel is going to be much more cost effective.
And you are right. Some sort of a frame sounds like a good idea. Wood seems acceptable, but I assume you could build something lighter out of PVC piping or something similar.