I had a boatshare arrangement for my first RIB. Three likeminded friends, equal shares on a new boat/used engine. (BWM DS21). It worked well and we never fell out. We had a master spreadsheet where we recorded maintenance/equipment expenses and every so often totted up who was ahead and who was behind in spending money and settled up. Running costs - fuel and two stroke primarily, were split between whoever was using the boat on the day.
Our arrangement was a little different insasmuch as my friends were less experienced boaters than I so effectively I ran the boat - arranged/carried out maintenance and servicing and they paid their share of the bills. I often ended up physically doing much of the work but I also used the boat far more than either of them. (I dont think they ever took the boat out without me!) so I effectively got most useage of a boat but only a third of the purchase/running costs (excl. fuel). Marriage/children/not being able to dive anymore meant that neither of them was able to use the boat as much as they envisiaged at the outset.
We sold up after 3-4 years and split the funds equally. No fallouts and still all great friends. I would recommend a statement of principles/agreement, particularly on cost sharing and who does what, but would have no hesitation on doing it again with the right people.
In fact I think at one time I tried to persuade JCW to buy a half share in Blue Ice
but the size of the tow vehicle required was an issue. For anyone looking for a half share in a very tidy RIB in the Essex area I would suggest they talk to Bern about La Bamba. He's a thoroughly decent chap!