If you want a fan that will move a lot of air and it's very inexpensive. Try an automobile electric radiator fan they are DC 12 Volt so you can chane the direction on air flow. If you want to control the speed install a reostat. I am going to build a frame and install mine in the forward cabin hatch.
these are extremely high amp draw and very noisy... I installed a 10" radiator fan in my atv trailer as a bathroom fan.... and it will hide any and all unusual noises coming from that area, and keep the air as fresh as a daisy... it will move a LOT of air very quickly. (i need to put diodes in the line to lower the voltage)
but I have options to charge from with my trailer, I have solar, i have a generator and I have my pickup... and sometimes shore power.
the rheostat will still cost you power, as it is only dissipating the power you arent using into heat....its better to put a couple of diodes in the line, as the diodes will lower the voltage about .83 of a volt for each diode.
the manufacture says to make sure the wiring can handle 25amps, and to use a 20amp fuse.... these are not good numbers for use on a boat.
on a boat you need something that is not power hungry, or that will only be used for short duration's and infrequently.
for the bathroom fan on our 34, I used a 4" ATV radiator fan, with diodes in the line to lower the voltage.... it isnt as noisy as when there is full power to it, but the amp draw will still probably be at 3-5.... with this much draw, its not acceptable as a cooling fan, but well worth the cost to vent the head when there is some serious thinking going on in there