There's a bit about this on the Fountaine Pajot Mahe 36
You can clear the intake from the engine
compartment and is fairly easy.
Turn the drive leg seacock off.
Remove the hose from the drive leg to the water strainer.
BEFORE OPENING THE SEACOCK make sure the seacock is above the waterline. If not, make an extension using some tube or hose vertically from the seacock to above the waterline. Then you can open the seacock and using a piece of stiff rod (even a straightened coat hanger will do) insert it down through the seacock into the leg. Move it up and down fairly vigorously to dislodge any growth which should then fall to the bottom of the leg. If the inlet holes on the leg are clear of growth then most of the dislodged growth will hopefully fall out of the leg through these holes. When finished I also blow out the intake as hard as I can with a few big lung fulls of air. Reassembly is the reverse and I prime the raw water strainer ( making sure it's lid is well sealed) before opening the drive leg seacock.
Make sure your diver knows that there are inlet holes on each side of the leg AND another one right at the very bottom of the leg too. You may have to repeat this process a couple of times to clear the growth adequately but it has been very successful for me.