From the Practical Sailor artical, it appears the Yamaha 6 & 8 share the same block. If you decide on it, I'g go for the greater hp.

The other engine is made by Tohatsu and is badged as Merc's and Nissan's as well. I really think the Tohatsu 6hp is a good choice for our boats (however it's badged), unless you have some really wilde tides to fight. And at 55lbs, it's relatively easy to handle. Down side may be the single cylinder. I know a single cylinder makes for a lot of vibration in the 2-cycles, but I'm not sure about the 4-cycles.

As for flushing, there is a removable bolt the hole of which takes a hose fitting. Got one on my Merc. You could probably use the Merc accessory in any of the engines. Pain to R&R after every sail, but is worth it for safety reasons. Besides, I don't have room for an engine bucket