As previous posters have stated you may have solved
it or not. It does seem like your raw water flow is too low if it took more than 30 seconds to empty that 1.5 liter bottle. So I'd look at water flow issues first.
Close the RW seacock and remove the hose from the RW strainer. If the hose is long enough to go above the waterline, great, rod out the seacock with a length of tubing or rod. Perhaps the through hull
is blocked by growth.
Check and clean the RW strainer.
Remove the cover from the RW pump
and check the impeller for missing vanes or simply replace the impeller. Also make sure that the cam plate is in place and the scoring on the inside of the cover and pump body is minimal.
Check that the coolant is full and clean and if you can, pressure test the coolant side of the system. The pressure test will show any leaks in the heat exchanger tube stack as well as any other leaks.
Inspect the mixing elbow
blockage by removing the RW inlet hose and rodding the passage
out with a welding rod or similar.
If all the above checks out well then the best way to clean the RW side without disassembling the heat exchanger is this. Buy 2 gallons of either Barnacle Buster or Rydlyme, a small bilge pump
where the outlet matches your hose size for the RW pump and a five gallon bucket. Disconnect the RW discharge hose from the exhaust mixing elbow
or riser and route
that as your return to the bucket with the Rydlyme. So you've created a closed loop for the RW side of the cooling
system. Remove the RW pump impeller and reinstall the cover. It's a good idea to slip a nylon panty hose over the bilge pump
to keep the crud from going back into the heat exchanger. Start the small bilge
pump without the engine running and let the Rydlyme circulate through the system for an hour or so. This will clean the system of most everything except impeller vanes and broken anodes. Of course this method does nothing for the exhaust mixing elbow.