We have lived aboard for two years. We spend May to October continuous cruising and are in our marina mooring in the winter.
Important things:
Enough bedding for you and guests and a spare set for each (that's four sets if you have two double beds)
Same goes for towels and have washable bathmats and rugs in the boat
Clothing - a few things for outings and special occasions/holidays elsewhere, otherwise be practical with what you need for boating
Outer wear - warm coat/jacket, waterproof gear (we use long fishing macs when we cruise in the rain - they fit over a coat if it's cold). Wear layers as mentioned in another person's response. Keep your favourites and lose the impractical stuff.
Shoes - make sure you have two pairs each for boat for when one gets wet and needs days to dry... otherwise it is your preference and allow storage for the fancy ones (I have loads).
Kitchen - We use things that can have more than one use... i.e. cheese grater is small and flat and doubles as a garlic grater instead of a press and the stainless steel steamer (holes in it) doubles as a colander. Our only electrical gadget is a toaster that we use when on hook up or the engine is running. We use a whistle kettle on the gas cooker and hand beat anything requiring such manipulation (whipped cream).
Tools - cram in anything you can, they are always useful
Ornaments - have them if you must - we got rid of most of ours because dust comes from everywhere (in hours it seems) and my husband drives like a maniac hitting everything - breakage is heartbraking so don't have anything too valuable unless you want to meander around the canals like most normal people do.
Look at your boat design and put as many cupboards in as possible - we had special ones made under the gunnel rails which are great for books, papers, and all manner of spices, some cans and so on. We have some useless shelves in our boat - corner thingys that are only good for ornaments and not much else... wouldn't design them into a custom boat - cupboards are best to hide the crammed in goodies.
Our great failing is we have loads of alcohol on board from cheap locations such as Spain - we store most of it behind the washing machine where we found a huge amount of unused space
Yes - a washing machine is a must. We have a dryer but only use it on the electric hook up. Although it will run when we are cruising, we do dry most stuff in our front cratch (need a cratch cover for that one), where it is strung up over a folding linen rack or hung on a hanger from a rail along the top of the cratch - also use one of those tiny round things with pegs for socks and panties.
We bought CD holders from pound shops and chucked all the CD cases away - can get a lot of DVD's and CD's into a very small space that way
We bought a small 12" laptop and got a T-mobile dongle thingy that connects us to the internet everywhere in Britain (even in Peak District) for £30 per month - it is fantastic.
We have TV but found the built in antenna useless. We got a regular antenna from Wilkinsons and put it up in an umbrella stand with an extending pole whenever we stop. Combined with a freeview set, we can get TV almost everywhere - including History channel and Film 4 if you are into that kind of thing. We found Satellite a pain because of the boat movements - only works if you have one of those expensive ones that constantly readjusts to zoom in on the satellite. The TV does tend to zap the batteries a bit so we switch it off (not on standby) when we aren't watching. Also use a wind up radio that works when the batteries run out and you still want to listen- we are addicted to the Archers (poor us).
We have a chargeable torch but also a wind up for when we forget to charge the big one or it runs out (while you are fixing the engine or something).
Our living room chairs are vinyl with footstools(leather look) so we can wash them down - seems to be a constant job with the dust. It is a bit like living outdoors inside really, so expect lots of airborne dust, leaves, spiders and insects. They aren't terrible, just a bit more than in a sealed house!
Our boat was a standard build cruiser stern - not a custom build like yours and we would have liked to have some prisms in the roof for extra light. We would also have had a bigger water tank (ours is 160 litres and we refuse to compromise on water use - fill up every two days). Most would say our tank is enough for a week - no way, I like being clean! It is stainless steel though and we use a filter for drinking water so get no funny taste or anything like that.
If you will be power hungry like us (tv and computer addicts) you need a good inverter, a good bank of domestic batteries and an engine size that will generate it all. We have a 2.5 kw inverter (only just enough), a beta marine 38 (only just enough) and a bank of 5 domestic batteries, one starter battery and two bow thruster batteries (only just enough) on our 58 foot narrowboat.
Last note- be as organised and as ruthless as you can with paperwork, bills, papers, mags, etc. They take up loads of room if you keep them. We have started scanning a lot of dated stuff (computer) and only keep the most crucial stuff in paper form.