Water cooling of small electric motors is a complex matter involving the transfer of heat from a source to another medium
Specific heat transfer of metals is a huge influence, together with water and air [literally] intermingled
If you have a jury-rigged setup, run the motor load until it is hot, feel the motor casing [$15.00 digital laser is helpful], connect the cooling water supply & repeat the same test
If the cooling water discharge is warmish, then you certainly have provided a means of cooling the motor internals
Both brushed and brushless motors can experience a significant heat as generated from the product of work
Most of the direct slip-on motor shell coolers with o-ring sealed ends appear to be anodised aluminium, some appear that the anodisation is an external process, with raw metal internals....so you would need to understand any potential reaction between your cooilng water and both the motor external sleeve and the internals of the sleeve cooler
I could only suggest you talk with some high powered brushless boat builders with onboard cooling, and work backwards from there
Keep us posted with your results
Derek