Over the years I have really grown to distrust and dislike external intake strainers of either the scoop type or the non directional (sailboat style) round type. The real danger in these devices comes when you have a blockage and can't get to the crud to clean it. They often require a dive over the side with a coat hanger but that does not always work.
To ream out an intake from inside, some pre-planning can make this a dry and easy task. It can be done in under three minutes if set up correctly.
1- You need is an intake hose that can be held up higher than the static waterline.
2- You need a snake or a BRT (blockage reaming tool). Mine consists of an oak dowel cut to the correct length.
3 - If a blockage occurs before the internal strainer, close the seacock, remove the inlet hose from strainer, hold the hose high and straight above water-line, open the seacock and ream it out with BRT...
The harsh reality of external strainers is that they can become can be a safety hazard, if they have not already. I find when intakes are plumbed properly there is little to no need for any sort of external strainer. You can far more safely ream out your intake from INSIDE the vessel if you don't have an external strainer and this can be a huge safety margin when you don't have to physically get into the water. In just one season here in Maine I was in the water three times with a coat hanger...(cussing)
If you're still concerned about clogs in teh intake, and not noticing them, you can insert a flow detector alarm on the intake hose or just pay close attention to your exhaust note. When it starts to sound a little hollow you'll know you have some flow issues.
Below you'll see some of the reasons why I don't like external strainers, some operator error, and some strainer error.....