What happens if?
 

If the belt for the water pump pulley is on too tight what happens? can it stop water from going into the motor?

???
no!
but if it's really tight, after a while it can cause force on the eccentric shaft bearings and stuff them up.

more likely to wear out the bearings in the water pump and alternator. these bearings are not as durable as the motor bearings.

you will have pre-mature water pump/bearing failure