I have a 4.3L Mercruiser (bought in 2003). I was removing the drain plugs
but could not find one of the plugs.

I found the one on the pump hose, the starboard side exhaust manifold, the
port side exhaust manifold and one on the port side of the block. I found 4
altogether. The manual says there are 5. However, I did find a hole at
the bottom of the block on the port(ish) side that did not have a plug in
it. When I attempted to put one of the other plugs in, it fit perfectly.

I did notice my boat was taking on water (unexpected water) during the
season. I did not notice my bilge pump working unessesarily. I trailered
my boat every time out and usually had several inches of water in the bilge.
It seemed to be clean water.

My questions are these:

1) Did I miss the plug altogether? If so, can you give me a way to find
each one of them (mainly the non-visible ones from the top of the engine)
I'll have to go back in and drain it before the freezing sets in.
2) If this hole in the bottom of the block is actually the drain plug hole,
how did I not notice a huge build up of water in my bilge? Could the bilge
pump keep up with the output of the drain plug?

I hope I figure it out before winter comes!

Mike