Few strainers will keep out fine silt, but that's not necessarily what you're worried about. Most will keep out the coarse, gritty sand that will do damage and clog things up. Since they're likely to be mounted below the waterline, don't forget to install ball valves between the pickups and the strainers. This way, you don't flood your bilge with sandy grit when you take the lid off to clean them out. (Ask me how I know...)
Also - When you shop for strainers, bear in mind how you will run your hoses, and how easy it will be to get the lid off and back on while standing on your head in a boiling hot engine compartment while the boat's rocking in bad seas and the Missus is getting impatient.
Here's my current setup, using modified Gil strainers. I relocated the fittings myself to get them where I wanted them. The only thing I don't like about them is trying to keep the o-ring in place on the lid when re-installing it. The smaller hoses coming out of the front are engine flush fittings. I located them under the screen, so that flushing water through them with the engines off should (theoretically) push everything that's on top of the screens back out through the water pickup. Since I have an onboard water system, I hope to be able to clean the strainers without having to even open them up. Kind of like back-flushing pool filters....