Managed Boat Schemes and Private....

Managed and un-managed boat share schemes

A List Of Management Companies Etc. 

The following firms offer shares for sale and manage the boats on behalf of their owners. Of course they do not do this for free and change a fee of ABOUT £300 per share per year (correct and approximate 2018) and if you ask them to sell your share they will normally charge a brokerage fee, perhaps based on a percentage of the sale value achieved, or just a flat fee.. They are listed by order of size (most boats managed).
Click on each to go to their web sites, but there is a brief résumé about each option at the foot of this page. 
Carefree Cruising (Sorry you will have to use Google for a link)
Ownashare Cruising
J D Boat Services
Our Narrowboat (one boat)

AND, of course there are

Private Schemes (see below).

There are also some timeshare styled companies. If this style of investment interests you please search on the Internet for them, the main difference being you do not actually OWN a share in any boat, and you can "bank" your weeks and use a different "resort" if you wish. At first glance I would suggest if you really want to go boating (and not jetting off as well) then boat sharing is CONSIDERABLY cheaper.
A little bit about the firms above
Basically formed out of the wreckage of Challenger Sydicatships when they folded in 2008 (and the MD was imprisoned for fraud). Some of the founders were ex Challenger staff. They manage around 55 boats (2018). Some of the boats they manage are ex Challenger, some are new (BCBM) builds and some are ex-OwnerShips boats. When OwnerShips collapsed in 2008 BCBM were well placed to take over the management of their boats should the owners want a replacement management company.
Carefree Cruising
As at 2018 they have 20 boats on their books. Founded in 2002 growth has been slow and steady. 

When OwnerShips went bust in 2010 shortly after the death of founder Allen Matthews Ownashare was formed by some of the staff there, however all have since left/retired. They manage 17 boats, some are recent builds and some are former OwnerShips craft. This company was set up some six months after OwnerShips folded. This is actually now part of Carefree Cruising. 
J D Boat Services
They had around six boats in their scheme but only one remains (Correct 2023). 
Our Narrowboat

Just the one boat is operated by them - or actually may now have none - best to ask them before you get in too deep!
and of course.....Private Schemes
A private boat is one where the group of owners run the boat themselves as a cooperative. Usually one person is treasurer, another bookings, another maintenance etc. If you are looking at joining such a group best to ask what the division of management is!

Some of these boats were simply groups of people getting together and starting a scheme, some are where one owner has decided to sell shares in their own boat as they were not really using it enough and a lot were ex Challenger and OwnerShips' boats where, when these firms folded, they decided, as a group, to "go it alone".
Whilst OwnerShips were the "big player" when it came to shared boats "Private" has now outstripped all other options put together and there are around 100(correct 2023) private shared boats on the UK waterways (+ a few abroad). Most of these are, collectively, ex Challenger and OwnerShips boats.