I recently immersed a small, 10 volt D.C. motor underwater for experimentation purposes. When I turn on the power, always to the same power level, it SOMETIMES spins effortlessly, and other times it doesn't. I had believed that it would either work or not work, period, without any "sometimes" being involved. The water used is always the same, in the same container. No variables at all exist, as far as I can determine. Any ideas why this is happening, and what if any variables am I not taking into consideration? Also, if I want it to always work underwater, what must I do?
> ... spins effortlessly, ...
Only for some limited values of "spins" and "effortlessly".
If there is water around the rotor it must be being displaced by rotor motion. Rotor shape will have an immense affect on power taken and so on speed. Your motor is effectively an un-designed pump or stirrer.
You must provide more details for any sort of especially meaningful answer to result. .
Keeping water away from the whole motor is the only "sure" solution.
ie use a seal that the shaft passes through with the motor dry.
If you want long term sealing the rotary seal MUST be designed and not "just happen".
This can be a complex task depending on specification.
More information is needed for a useful answer.
If it is a brush type motor (as most small cheap ones will be) you MUST keep water away from the commutator at a minimum.
A brushless motor could notionally work with water around the rotor.
Bearing life running wet will be an issue as will corrosion.
Any electrical connections must be insulated (as auchmonoabspielerbar notes).
This site might help. I do not know the exact type of motor you have: brushless vs brushes. Although the site does not talk about immersing the motors it describes the general principles on which it is based. The motor is not designed to work underwater and would be terrible for the purpose. Corrosion would end in the motor failing. For the longest lifetime underwater use a brushless motor.
It really depends on the water. If it is de-ionized or distilled water a 10V motor will probably work underwater though the "windage" now becomes water resistance so you will loose a lot of torque.
Using it in salt-water or sea-water would be a really bad idea and will release chlorine gas which is deadly.
As for why it starts sometimes and not others it is difficult to say. Perhaps a film of water is forming under the brushes breaking contact of increasing the contact resistance, or it may just be the increased drag takes the must start voltage that much higher.
Over time the thing will corrode and that will change the conductivity of the water accelerating the issue.