I know my hull is in very good condition and doesn't leak but whenever I leave the boat moored for a few days and come back it has water in the engine room…..
I think I have screwed the bung in tight enough so that can't be it is it something to do with my drives or exhaust system???

also if it makes any difference it is often rough where I leave the boat

what could be the problem I have confidence the hull has no leaks any help appreciated

Hi way2fast you could have water coming in at the transom shield , if you can get in there check with a torch to se if you have a small trickle , could be a swivel pin seal gone bad ,also check hoses especially the one befor the water pump .

I had the same problem and it was the sealant went bad around my thru hull transducer.

The list is long but first thing I would do is try a bit of Teflon tape on the plug threads of you haven't already. I know they are tapered threads and are supposed to self seal but I had a boat that no mater how tight you got it would not seal I spent a lot of time looking for a leak then one day just for the hell of it put some tape on the plug problem fixed. If that's not it go to checking the transom assembly. They like to leak at the steering shaft seal so look where the tiller arm goes into the transom

shift Bellows, drive shaft bellows or steer shaft seal,,,,, pray its the bellows

I noticed the same thing with my boat and I kept checking for leaks, etc. Then I inspected the water closer and it was fresh water (boat was floating in salt water).
The light bulb went on and I realized I was getting water in through the vents when I washed it off.

My built in ice chest drains to the bilge. I figured it out after I found lettuce in there.