I just posted this to another forum. It fits your situation:
There are two ways to do it: recirculation with an auxilliary pump, or filling
First hook up a garden hose to the A/C raw water
outlet and backflush to get the
bulk of the loose stuff out and make sure that you have a good flow. Disconnect
the raw water pump
before doing this and let the water run back into the bilge
For either the fill and soak or recirculate method, use a 1:2 solution of
Rydlime or CLR.
If you fill and soak, disconnect the raw water pump
and raise the hose to the
A/C as far as it will go. It must be above the A/C and if not splice on another
Then using a funnel, fill the hose with the solution. It will work even better
if you heat it to about 120 deg but be real careful. Fill until it overflows
outside. Tie up the hose and let it soak for a couple of hours. Then hook up the
raw water pump and turn it on to flush the solution out. You will be amazed at
what you get out of it.
To recirculate, use a small, general purpose pump and hook the discharge to the
raw water pump's outlet hose. Hook a small length of hose to its suction and
drop it in a bucket. Then run a hose from the raw water outlet of the A/C back
to the bucket. Fill the bucket, turn on the pump, add solution if necessary to
maintain a half level in the bucket and circulate for an hour or so.
When finished, reinstall the raw water pump and the A/C raw water outlet hose
and turn on to flush.