Oil gone?

Hello all.

Today was service day for our mastercraft prostar 190 1989.
We bought a new oil filter and some oil.
Our boat is stationed in a harbor. (yes, its always in the water, we dont trailer)

we had a pump, so we could pump the oil through the oil level measurment tube. But first we heated the engine by letting it run at 1000 rpm for about half an hour.
The oil carter was hot and so was the oil.

We started to pump and about 2 liters of oil came out. We were like "hmm 2 liters, that's not much."
So we putted a little bucket in the bilge and loosend the drain tube. Nothing came out so we though "Hmm it maybe was already low on oil. Let's fill her op with clean oil."

And so we did. We started to poor in clean oil and kept checking the oil pin. After about 2,5 liters of oil the oil level was again back on the right hight.

*So my question is, how is it possible that only 2 liters of oil came out? Cause it's a 4.5 liter carter.

*If we didn't get all of the old oil out of the engine and it's now mixed with new oil. Is it bad to leave it that way till next year?

*Is it possible to get all of the oil out when the boat is in the water? or should it be on the trailer?

Thanks in advance.

For the rest.
It's about an MC 190 Prostar building year 1989 with a ford pcm 351 engine.
And ohh yeah, i'm dutch so please excuse me for my bad english vocubalery