I ran a 1/4 inch polyethylene line out the transom.
I simply drilled a small hole, then I glued in a piece of 3/8 copper tubing.
Passed the 1/4 inch line thru it and slightly crimped to hold it in place. Likely also sealed it with permatex gasket maker
Engine side, I tee'd it off from the raw water injection, I brazed up a copper tube to an existing raw water part, then passed in the plastic tube and sealed it with Permatex gasket maker., Or I used a short piece of rubber hose, can not really recall..
I used clear poly tubing like you can buy at Lowes as it is very cheap around 11 cents a foot..
Did that at least 5 year ago.
I did this on my main engines as I have underwater exhaust extensions, and you can not see the raw water flow.