HI Nick, this is a bit of a long reply - sorry !
For what you probably do, your insurance is perfectly adequate, indeed many insurance companies won't give comprehensive cover for home 'built craft'.
You will be covered for damage and injury to third parties ( and hopefully passengers ! ) but with no cover for damage to the craft, yourself or any legal costs if your craft is damaged by someone else. With a home built craft that isn't a massive issue as you would likely do the repairs yourself and not make a claim ! It is the third party cover that is the important bit as that could run into millions and you will probably have a £5m limit which is generally reckoned enough. Until recently that was £2m but has now been increased. Basic Boat insurance also covers operation on land if part of an organised club event ( as far as I can recall from my Sev Vanguard days ).
Comprehensive cover costs more but is like car insurance and similar to ordinary commercial marine cover. You get repairs up to an agreed value for damage or if a constructive total loss ( aka write off ) or a full agreed/market value pay out for loss due to sinking or fire plus cover for claims made by passengers as long as not commercial. They will also cover costs for agreed salvage and for personal items lost. It is actually more complicated than this due to the %age of value on cover but we can ignore that bit. You will also have an option of legal expenses cover for you and to reclaim any lost no claim bonus or to defend a prosecution against you. There is also personal injury cover and a good company - I am with Haven Knox Johnson - will take over the damage claim and get the vessel fixed and returned just like a car policy. You may even get a hire vessel - some hope with a hovercraft ! - during the repair period. HKJ asked me to rewrite their hovercraft insurance terms some years ago and they still use the wording.
There were a couple of 'spurious' claims a few years ago when some commercially built but defective craft were 'damaged' and the insurance paid out. No names !!!! However many companies took fright and dumped all hovercraft insurance, but a few still offer it although with special clauses and limitations. There are so few of us that they don't understand the mind set and a few maverick operators can cause the underwriters to back out. Easier to say no than to handle an unknown risk.
The main fear for insurers is that of Road Traffic Act liability and this is an incredibly expensive and dangerous issue for light hovercraft operators. The liability is unlimited so in the event of an accident on land, any claim company will try hard to make out that the hovercraft was being used in the manner or purpose of a 'motor vehicle'. That way they can claim huge sums and take their cut. If the insurers won't pay out, they could go after and potentially bankrupt the operator.
With any publicly accessible space now being classed as part of a highway for the purposes of vehicle use, this is a minefield and why Road Traffic insurance is specifically excluded on hovercraft policies. It is also a big risk for hovercraft operators on a beach as it could be argued that you are a motor vehicle - certainly a craft operating on a beach with no intention of going on the water might come into this category. The wording .... from memory ....says 'adapted or intended for use on a road' but there is a massive grey area and that is where lawyers make their money. A fully equipped seagoing hovercraft heading to or from the water or below the mean high water mark is unlikely to fall into that category, but a basic one using the beach as a playground might, and lawyers are trained to find a chink in the armour.
Basic Boat insurance, as I recall, covers operators for use on land ONLY if part of an organised club event. My insurance lets me choose - both obviously subject to the RTA exclusions - and that's why I would need the club to make any hover-in an 'official' event so that anyone with Basic Boat insurance is covered, along with the organisers as club officials ! Not a problem if they decide not to do this, but it shuts out members without comprehensive insurance cover.
Sorry for the long pontification !