I replaced the fairly useless UFO vent above the head with a solar vent (stainless steel model) . It's brilliant. Gets rid of odours fast and always leaves the head fresh when the boat is closed up.
I bought the day and night model off Ebay for around half the Whitworths catalogue price for the Sunvent model. However, I think that the more expensive Nicro vent that Whitworths sell may include a screw-in plate to seal the vent in rough weather. This would be a good idea but I figure that short of a complete roll over a bit of water sloshing into the head is no big problem.
I have also bought a stainless steel mushroom vent to be installed above the stove. Generally I don't like mushroom vents but as I use the stove in winter to heat the cabin I wanted something I could close easily to avoid heat loss.
For extremly sea worthy ventilation Sven Yrvind has some origional ideas
http://www.yrvind.com/present_project/?m=200808Cheers
Keith