Any water prop set up is going to be an amperage hog unless you go low kv motor and large sub surface prop, which wont be very fast.

Don't get discouraged, you have a very unique situation.

I know, can you share a link of such motor/prop?
I looked at some forums and i do see that wet props give way more push power for same amp comparing to air prop (Airboat), my only concerns are weed get stuck inside the prop.

Well yea! Like Crispy said your idea is not NORMAL! Airplane electric motors are not designed for water either. Thats what makes this hobby great. You make it happen with hard work and effort. If you want easier look a giant scale brush-less airplane motors. Buy two and put them side by side or inline. Buy a crap load of batteries. Throw it in the water. I can bet whatever you do, you will rebuild to make it better.

So i want to get it straight - A wet propeller is more efficient then air propeller when it comes to a floating object movement?

I have little knowledge of RC craft but am learning but it seems to me if you wanted a n electric craft or airboat to run for a long period of time and without anybody having to control it, you take suggestions already made here with a few modifications and solve the problem. I would either put a central pole or anchored floating station in the pond. Run a wire to the pole powered from 120 ac with a 12 volt dc converter to power the boat motor. Attach the air boat to line with a small power wire attached to the line to power the motor and horn. Or If the boat has to run in a straight line out and back 2 poles and run it like an electric trolley car but on water. But keep in mind I know very little about the RC industry yet.