I have owned two boats over the last 30 years with external strainers. With the first boat I removed the strainer each spring and painted the interior surfaces with antifouling, after removing last year's paint, and never had a problem with growth at all. When she came out in the fall the strainers were clean. When I took delivery of my current boat new, there were no external strainers on either the generator raw water inlet or the main engine inlet. Strangely, however, the A/C inlet did have a strainer external. The first weekend I used the new boat the generator shut down twice for temperature when the inlet became clogged with floating vegetation and I dove twice in 50 degree water to clear the clog. Suffice it to say the following week I had her short hauled and installed screens on both the main engine and generator. Since then I simply spray the interior of the exterior screens in the spring with the same anti-fouling stuff I use on the prop. No problems. Here's the thing, sure you can clear a clog internally if you don't have an external screen but I guarantee you will not want to do that on a dark and stormy night making harbor under power when you suck up a bunch of grass into your main engine inlet and she begins to overheat. When I need my engine I want it to be there, period. BTW, when I was installing my screens I was debating screen forward or screen aft. I was aware of the ABYC standard of screen aft on sailboats, but I wanted to see what boats on the hard had. Naturally I looked at Hinckleys and they all had external screens facing aft for the main engine. Good 'nuff for me. That said, it is critical that the screens be maintained, like everything else on our boats.
And that is why I no longer use external screens.
In 1988 on a Post 50 Sport Fisher the generator sucked in eel grass and shut down. Took over an hour at 1:00 am to get it running again. Impeller had been toasted and the eel grass clog, even with full dive gear, took nearly 40 minutes to get free with coat hangers.
It then plugged up the next morning at about 7:30 am again..
In 1996 on our Cape Dory I experienced the same type of clog heading up into Robinhood cove. Not a good time to lose the engine. Again, it took over 35 minutes with coat hangers to free the clog while diving in 3+ knot currents.
In 1999 on our Catalina 30 we suffered the same fate only this time drifting towards ledges with no wind. Got the drifter up and missed the ledge by about 30 feet. Spent over an hour getting my head banged on by the hull trying to free the eel grass. Took over 35 dives holding my breath to clear it.
In 2006 I experienced our first and only intake clog since removing the screens. Had it free and clear from inside the boat in under 4 minutes. I can't even put my we suit on in four minutes... With our current set up the eel grass makes it into the large internal strainer which I can open and clear in well under two minutes....