I have two plastic zip ties and a small combo lock, one is connected to the overboard discharge thru-hull handle and the other to the intake thru hull base, the two are a few inches apart (although any hard point will do, if you do not have one just affix one to the hull with 4200, 5200, or
epoxy) so when connected with the lock the tank thru hull cannot be opened, I received the requisite compliance sticker, so I assume it is ok. The language is very broad. People usually run into problems with the Y valve and the overboard seacock not being 'locked' but only closed.