From a small dinghy to a large super-yacht or a super-fast Jet Ski, each type of vessel will attract a different level of risk, so just like car insurance - boats are usually categorised according to their vessel type and risk.
It goes without saying that a Sunseeker with powerful engines that costs a million pounds will be far more expensive to insure than a smaller, much less powerful craft.
A yacht will also have a different risk assessment than a dinghy or a jet ski, so insurers will factor in the type of the vessel and any potential repair bills before calculating the premium.
In general, the actual size of the boat isn't as important as its value, how easily it could be stolen and the potential for damage. If your boat is trailer-able and spends a large amount of time safely tucked away on your drive, you'll probably pay a different premium than if it was stored at a marina.
The area in which you keep and use your boat will also have an influence on the level of premium you pay and may have restrictions on cover that are subject to local weather conditions.
Finally, the experience of the owner / users and the type of use i.e. whether it is used privately, as a charter or racing vessel and of course any previous claims experiences are all taken into consideration when calculating the premium.