To start at the beginning, where the water comes in, at the deck fillers. The seals of the deck fillers were shot, and no longer available. To seal them, I'd have to bodge some sort of gasket. And the screw holes around the edge allowed water to leak into the deck. So I dumped them and sealed up the holes in the deck. To fill the tanks, you now need to open the aft hatch lid and put the hose into a pipe that leads to the tank. The through deck fitting has been eliminated, eliminating damage to the deck from leaks, and eliminating the chance that sea-water could contaminate the fresh water.
The tanks themselves were fibreglass and tainted the water. You could fill a tank with water one day and your tea was tasting of styrene the next. I fixed that, documented here.
Pipe-work was mostly hose-pipe and jubilee clips, a terrible choice. The shower was a miserable trickle, and I thought I might dispose of the electric pump. However, on swapping all the pipe for domestic plastic fittings, I got rid of a bottleneck and found the shower was now rather good.
The shower is cold water only. I didn't fancy having a hot water boiler on board for all kinds of reasons. I don't like cold showers, but sailing south solved that. If I intended to sail in colder places, I'd alter the pipework a bit so that the pump supply can be switched suck from a container rather than the built in water tanks, and the container could be filled with warm water supplied from the stove. It's a little work, but less I imagine than the hassle of maintaining a boiler.
There used to be a bilge pump under the shower floor, but a catamaran doesn't have a proper bilge, and so there was always a bit of water left, which when ignored went smelly. So we switched to scooping out the water with a dustpan and emptying it down the sink. Takes less than a minute. Simple.
There are no through-hull fittings under the waterline. I was considering removing them when I was first taking out the pipework of the old toilet. A family emergency called me away, and when I came back, the port hull was full of water from a leaking through-hull. I decided then to dump them, and fill the holes. I don't regret that.
The rain catching system I installed is very pleasing.
In my blog of sailing the Atlantic circuit, there are accounts of gales and calms, video of whales and dolphins, and accounts of curious things happening in tropical places, but always and still, the account of installing a compost toilet gets the most hits. I wouldn't change it at all. :) If you read that post, note that I gave up on the charcoal and have become less mean with the covering material. Problem solved.
The bathroom has pumped water for the sake of being able to have a shower. The kitchen has a foot pump, so that you can most easily control water use.