I thought I would offer my expertise may be a little help.
I have dealt with many water contamination situations.
Mostly on automatic transmissions when the transmission cooler ruptures inside the radiator . When the radiator gets hot and pressurized it puts water into the trans and usually in the morning the transmission pressure puts atf into the rad.
Once this happens you have a strawberry milk shake and it is extremely hard to get clean up.
A story
I had flushed out a van transmission and thought i would take it for a good ride and when it got hot the water in the fluid started to steam and blew out a lot of atf and left me walking.
So i usually do two to three complete fluid changes (24 plus quarts of used atf) then drive it hard to steam out any water that is left (DO NOT DO THIS WITH MOTOR) then i do the another complete fluid change with new atf.
On the rocket is there a coolant tube that runs through the motor that could be damaged?
If i were doing this job i would find a way to pressurized the radiator to about 15 pounds and pull the bottom plate from the motor (you are going to have to do that anyway to get rid of the water and maybe a couple of times)
Anyway with the radiator pressurized and the bottom plate off maybe you can tell where it is leaking.