Ayuh,..... Welcome Aboard,...... Those are usually called a garboard plug sleeve,....
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2332490.m570.l1313&_nkw=T-H+Marine+SWD-1W-DP+Splash+Well+Drain+Tube&_sacat=0
Ayuh,...... Welcome Aboard,...... Merc started deleting grease zerts 'round '86, so I suggest you lube the moving points with yer favorite spray lube,.....
Ayuh,...... Test it just like for a cracked block,......
Drain it, isolate the block, plumb 15psi of air into the system, 'n listen,....
The leak should be obvious,... if not, spray the sound with soapy water,.....
Ayuh,...... Look closer at the trim pump,.... it should have it's own oil supply, as it doesn't use drive oil,....
It's usually Atf, or hyd. oil, or motor oil,.....