I don't use an isolator at all - non of my car's ever have, and it's one less thing to cause a break down. Particularly because I've found the commonly available isolators to be very unreliable. The reasoning is this:
There appears to be two reasons to have an isolator.
1 - Safety, should a short occur then you can isolate the battery. However, almost all of the electrical system will be protected by a fuse, and the remaining unfused wire to the starter should be carefully routed and insulated so as to mitigate risk of shorting. And I seriously doubt that anyone could react fast enough to prevent damage if this wire did short to ground.
2 - Electrolytic corrosion. If the positive is left connected around the craft, and salty damp conditions provide a path to ground, tiny leakage currents can cause corrosion in switches and other electrical equipment . This, however, is less of a problem with non-conductive hulls and not a problem when the craft is stored away from salt water.
Safety is always a balance of risk, and rarely is something unambiguously 'safer'. For example, a lanyard may prevent the engine running on if a person goes overboard, but conversely if it is accidentally disconnected when operating at speed at a high yaw angle (side slipping) then it will cause a capsize. You have to decide which risk is bigger, before you decide whether it's 'safer' to have a lanyard or not. A similar principle applies to the isolator. It could perhaps prevent a fire, but equally it could fail at sea leaving you drifting onto the rocks. My view is that the former is less likely, and the latter is more likely, and therefore it is safer not to use an isolator.
As the risk of capsize when yawing is peculiar to hovercraft, then this calculation may be different for a boat. The lanyard arguement is also different in a boat, we have all heard tragic tales of people being run over by thier own boats, something that is much less likely in a hovercraft (because the steering self centres) and much less likely to have tragic consequences (because there is no water propeller). So these differences should be considered when deciding whether it is safer to use a lanyard or not.
As often with these things, this a nuanced picture that may have different answers in different situations.
Ian