One of the key things to realise before embarking on insurance for your watercraft is to be clear about which type of policy you actually need. The way to differentiate between the two is actually fairly easy. A good rule of thumb is as follows:
Boat Insurance - Watercraft that are up to 26 feet in length
Yacht Insurance - Watercraft that are 27 feet and over in length
Once you have established which type you need (you will find that most boat insurance companies will offer both) then here are two important factors that you need to bear in mind before finally settling on which policy to get:
1/ Agreed Value vs Actual Cash Value Settlements - You want to be clear about this ahead of time, because an agreed value settlement is far more favourable to the boat owner, and an actual cash value settlement is much more favourable to the boat insurance company, because it factors in asset value when the loss, theft etc was ACTUALLY suffered.
In practice this is likely to give you a headache, because unless you are an actuary you are going to have a hard job knowing what the insurance would pay out at any particular point.
So, for a quiet life, and the sake of your wallet, you should always get an agreed value policy wherever possible.
2/ Deductible Amounts - Make sure that you are clear how much the deductible on the policy will be. This can vary much more than you might think both from policy to policy, but also depending on the type of accident that the boat suffers that requires you to seek a settlement.
For example, hull damage is likely to be a 3% deductible on a yacht, but windstorm damage is likely to be a totally different amount.
Make sure you know what they are.
The best way to proceed is to get yourself a boat insurance quote right now so that you can immediately get a feel for how much the policy will cost. In this way you will be better prepared, and can better gauge the differences between the policies.