The marina development opportunity is just that at the moment - an opportunity. Revenue is zero. Planning permission doesn't exist. Plans exist, a deep water quay and land exist, and a willingness from the Duchy of Cornwalll to lease the site exists, but that's about it.
The link posted by jimg is for a different business for sale that does have revenue etc....
Errm, I have been thinking about this for some time now. Not in the manner described here but looking for ports to develop. I was in some way involved (not financially) with the build of the small marina scheme in Porthmadog. It cost £135k to build and delivers £33k per annum revenue from 25 moorings. Not a bad return if you have £135k to invest. Just need to find a small port to develop...