What's the deal with the Merc P/N 811573A04 motor water pump for $396 ? Is there anything special about it ? Is there a after market that's equivalent ?

I've got a new one in box, that I bought for my 700's last year (just in case) - never used it, never installed it. If you need one, $350 shipped?

Marine Cir pumps have brass or bronze internals do to raw water (fresh/salt)
If your closed cooled you could use the auto brand

Also marine pumps will move water in ether CW or CCW rotating,