Once you get it cleaned, you may want to try this to avoid future flow issues:
I made a habit of reverse flushing with fresh water through the discharge as part of my winterization.
I removed the strainer and replaced the cap. Then I stuffed a suitable length and diameter of cheap clear hose in to the discharge. The end of the hose that goes in the discharge is bare, the other end of it has a barbed nipple inserted with a garden hose fitting on the other end. I attached a garden hose to this and put fresh water through it for a few minutes while holding hose in discharge.
Once finished, I removed the garden hose and followed up by hooking up a modified bucket to the short length of clear hose. The bucket had AF in it, and there was a cheap Harbor Freight type hand pump between the bucket and the length of hose that was stuffed in the discharge.
I hand pumped a couple of gallons of AF through the system until it came out nice and pink through the intake, then replaced the cleaned strainer, and I was done.
The hose set up, fittings, and hand pump were cheap, and once I had it all set up I used it every year.
The results of the fresh water flush was that I never had a flow problem in the 10 seasons I had the boat so I guess it worked pretty well.