and this is where I come out! (and I stress that these are the arguments as they apply to me not to anyone else. I offer them for comment on my thought processes such as they are and that they might interest anyone else!)
First question - Can you afford the purchase price?
Diesel boats have a price premium over petrol boats. Example I have is 6.3 Humber with 140 Suzuki 4 stroke - 17k (basic spec 1 man console + trailer) vs £22k for the same boat with the 1.7 Mercruiser diesel. Data from Humbers own spec sheet or my recall of it! Similar or larger differentials probably exist on whatever boat you buy - Ribcraft, Scorpion etc etc.
Putting aside the running cost issue for a sec. Do you have the wherewithal to afford a diesel boat? That may be not just do you have a large pile of cash sitting around but can you afford the requisite loans, mortgage or selling the kids for medical research!
If you can (you lucky chap) then the argument essential goes down to the fundamental - low tax diesel @ 30p p l vs fully taxed petrol @ 74p+ (£1 in a marina) p l. A relatively easy calculation of number of hours you will use it vs fuel savings will tell you whether its a good deal or not. Point I would bear in mind is that a cheaper to run boat (per hour) may well encourage more frequent useage! The flip side is that there are only so many weekends in a year!
Where life gets a bit more complicated front is if you try and rationalise a larger loan say on reduced annual costs. (i.e. it may cost me another £x per month for the loan but I will save £y per month in running costs.) Its about this point I either get scared about the size of a the loan or work out that I really can't afford any kind of boat and end up in a sulk!
(Or get nervous that the EU will force the imposition of full tax on marine diesel and the benefit will fly out of the window the day after I buy a diesel RIB!)
So thats the money issue. The other is inboard/outdrive vs outboard. Pro's for outboards are "designed for the environment they are in", easy(ier) to service, take up less room etc etc. Cons are of course the fuel consumption and availability of fuel afloat. Inboard/Outdrive Diesel - pro - fuel consumption, torque much improved availability of fuel & entertaining turbo whistle!. Cons - takes up lots of room in the boat, not so easy to service, maybe not designed for the environment its in (the mercruiser 1.7 being a vauxhall Astra diesel iirc) and increased servicing costs. And outdrives are expensive things when they go wrong I think that much is pretty clear!
So to go back to your original questions Mike, no personal experience of the Mercruiser (but there is a chap who posts here who has one in a 7m humber I think.). I do have some data from Humber that is pretty impressive which I can fax to you if you are interested.
Would I buy a diesel? Yes if I was in the market for a "Twister" or similar. (8.75 Scorpion / Yanmar 300) but until my lottery numbers come up I'm not. Would I buy a smaller diesel RIB vs a petrol........................................the honest answer is Dunno but thinking about it!