In fact, every battery pack must get hot, but all batteries equally.
Today I made run with my ECO racing boat. I use 7 cell GP 3300 Matched Pack, gold connectors, and silver battery bars. 18x2 Corally BB rebuilt racing motor and 29mm Graupner Prop. With this configuration I ran about 7min in full throttle and my battery pack was very hot. When you taking it out immediately you will burn your fingers. If one of the batteries is dead, it will boiling then.
Generally, electric boats are big power consumers and useful payload will be less than 10 minutes depending on motor and prop.
Now about diodes.
Diode is at least 9A Schottky and it purpose is to protect ESC breaking FETs against
reverse voltage and have nothing to do with battery heating.