John,
How about fitting a pump with a inlet hose fitted with a strainer and check valve so rather than use a thru-hull you just throw the suction pipe overboard and away you go. You would need to buy a self priming pump but they shouldn’t be much more expensive, I am not suggesting you use this ONE but a quick look shows they are available for around the £100 bracket. If you decide to go down this route I would suggest making provision to disconnect the hoses and store elsewhere when not in use.
Let me know how you get on,
Paul.