The raw water
"filter" is generally referred to as a strainer. It should be located very close to the sea cock or shut off valve at the thru-hull where the water
enters the boat. The strainer isn't a very fine filter. It keeps out leaves, fish
, twigs and so forth. It doesn't keep out small things like a fuel filter
does. Most raw water
paths are not very small so small stuff just goes right through the system and out the exhaust
Here are some typical strainer types: GROCO STRAINERS
Some boats don't have a strainer so it's possible you don't have one.