Yuck. I have the ones that Jdpber recommended on my T-4.3 engines in my 278 Vista, and they are
NOT fun to get at (at least in my bilge). Especially if you drop the wrench/socket/socket with extension/whole rachet with socket with extension down in the bilge.

Sometimes that happens multiple times. And the I start cussing in multiple tongues, lol.
I dont have these yet, but this style has been recommended to me several times by my marine mechanic:
http://www.ebay.com/itm/MERCRUISER-MARI ... 56&vxp=mtrThey are blue, easy to spot, and you can use your thumb (cant drop that in the bilge!)
I plan on buying some and install them at the end of a piece of short pipe so I can reach them really easily. Having twin engines and a cramped bilge with a generator and hot water heater, makes draining the engines a NIGHTMARE.