Actually it can be done for around £120 plus the sub to Sky for the chosen channels. We were next to a 24ft Mobo with one sitting on deck, quite useable in a marina, pretty hopeless on a swinging mooring I was told. The set came from a company specialising in TVs for mobile homes, so try a search.
I hadn't looked at that site for some time and hadn't seen the Broadband system.
Their satellite dishes are pricey though (but then so are their TVs), however I'm sure I saw a system for 12v from either Pumpkin or Force 4 or the like for around £120 (but then with an extra monthly sub for Sky) and it was one of those I saw a small mobo using in Yarmouth IOW last year and he said it worked very well. I think he might have said he used the card from his home Sky Box so no extra charge? It wouldn't help us however as we have cable TV so would have to pay Sky.
the eight grand unit is a motorised dish that adjusts with little motors up a bit sideways a bit as the time according to boat movement, and will track to keep a god pic even offshore as a planing boat bashes through waves. The mobile home stuff is fixed or semi-fixed hencellot s cheaper - tho wd only work in a marina , really.
If you're looking at Sky systems, you can pick up set top boxes on ebay for less than £40 and dishes including cable for about £15.
Then call Sky and tell them you've got a second-hand system and you want a contract and you can subscribe without the clause about being connected to a phone line for 12 months
Or you can ask for the free-view card which costs £20 and gives you about 100 channels.
Or, if you've got Sky at home (I know you mentioned you live aboard, but this is for others too), buy another set-top box and call them and say you've connected a second box and you'd like another viewing card. this will cost you only £10 per month regardless of your primary subscription.
As for alignment, we get no problems with the signal whilst in a marina at the moment.
When we were on the river bank tied to a barge, we had to adjust the dish occasonally to account for windy days affecting heading but nothing too serious.