Thanks for the nitrile rubber idea I'll get some and give it a go, after bathing my wells in vinegar. . . . . . .hh the wonders of white vinegar. Pretty much unchanged since 3000BC and cleans up just about anything, I think I'm right in saying that it will dissolve aluminium foil, better shrink wrap my saildrives before applying liberally
As to the purpose of the boot I guess it's purpose is to exclude the critters and improve water
flow over the hulls and yes you are probably right too much effort to remove saildrives just to clean out the wells, I'll wait until the diaphragms need changing.
Boat building is the amalgamation of incompatible products in the pursuit of perfection, all of which will be undone in time by saltwater alone!