x props have more pitch, they will give more torque, at the cost of higher amp draw, thus shorter run times. I personally only use x props, as I like boats, even scale, to have more torque and speed.
You then bring into play, the size of the blades, most common available plastic x props are perfect for testing, as they are cheap, but they only come in two blade format. It all depends on how fast, and for how long you want to run. I go fast, for short time, 10 mins if going round and round is enough for me, other will prefer slower running, and a much much longer run time.
Brushless and lipo would suggest you want some speed, otherwise why do it, so x prop would be the way to go. Start small, and work up. A warm lipo isn't bad, a hot one is! if you over discharge a lipo, and it swells, its finished. The heat is generated by the discharge rate, the pack will have a c rating on it, the higher the c rating, the better the discharge rate, generally 30c and upwards for a heavy discharge system, remembering that Chinese stated mah, and c rating are generally well over estimated .
The kv rating of the motor is also important, under 1000kv (as a generalisation) will give more torque, and therefore suit a heavier boat, higher kv will spin up more, less torque, more top speed, so lightweight fast electric racing boats will go this way.
If you buy a cheap amp/watt/volt testeer, such as this
http://www.4-max.co.uk/wattmeter-budget.htmyou can plug inline, and test the amp draw. Your motor will have a working max wattage, and more importantly your esc will have max amp useage, exceed these at your peril, you will end up with that smell of electrical death, usually in the middle of the lake
. Its the only way you can really analyse what is going on, your fastest set up might not be the best!