Advice was: if you Carry one , use it on land or on larger boat.
I would argue that is badly worded advice
If someone has stopped breathing and is unresponsive it would be wholly wrong to do nothing, if you have an AED on board then it ought to be integrated into the best quality CPR that you can manage given the conditions.
Head for help ashore or to an ALB of course, but immediate effective CPR+defb is your buddies best chance of walking out of hospital.
I suspect the issue for the RNLI is the devices aren't waterproof.
You can get some in a waterproof case but AFAIK no-one yet makes one thats even IPX7 rated. In addition defibrillation practice has evolved loads in the last 20 years. When I first trained on a defib 15 years ago I was told not to use it in a puddle. Back then the UK Resus Guidelines said a defib should only be used by someone with training. Nowadays while UKRC recommend training they specifically say you don't need to have been trained to use one. Times move on...
However, high quality chest compressions are also needed (50% of arrests are not immediately shockable), and high quality compressions on a RIB is not easy.
But the fastest way to get a defib to a yacht / motor boat etc out at sea is presumably by RIB unless its a long way out...
I had a look at a machine that the Coastguard Helo teams use. It was rated for use in driven rain (dunno what that makes it IP-wise). It was a stunning bit of kit - full defib, exhaled CO2 monitor, cardiac monitor, printer, the works.
They cost €25k a pop
This reminds me - I need a new battery pack for a Defibtech AED - any leads?
we carry defibs on the aluminium catamarans and a big thick rubber mat to put casualty on if we need to use them . No solid science behind the mat, just seemed a really good idea during the discussions.
Think the main reason about the Rnli only allowing defribs to be used on all weather lifeboats or the Thames boats is the saltwater ( electolite ) aspect of things with the lack of any shelter on the inshore boats .
once had experience of a saltwater immersed outboard engine that give a fair old shock to one of the crew from a plug lead that was tracking to the engine casing and he was wearing damp wetsuit gloves :0
I've had a hell of a shock from a dry fiat panda HT lead, and several from wet ones. (That car was out to kill me!) The voltages in an engine HT lead are huge! And the current has no route to ground other than through air (hence the spark in the engine)