How much watertight motors are?

Hi, was about to buy one of Maytech electric motor, watertight inrunner, bit bigger one (it is not a case). at the end i’ve asekd how much the motor is watertight, how long it can stay in water, ect.
Been told that those motors cant be use in salty water, my needs keeping it underwaer for 6 hours was to much, and after use i should open it to dry???
So how it realy is with those motors? all have such problems?
I was going to use it for tiny powerboat so more like trust motor - maybe in such case (as looking for robust waterproof solution is outrunner (sealed in epoxy)
thank you for any feedback