KBJ -- great question -- your answer depends, most of the time, on what other policies you might have.
As a general rule, umbrella policies "go over" other policies you might have -- for the most part that do not add new coverage. Example-- Say you have a house policy, auto policy and boat policy -- each with $500,000 in liability coverage. If you bought a umbrella policy with $1,000,000 in limits, that policy would come into play only IF the liability for a claim against the house, auto or boat was not enough to cover the loss. Say in the above example, while driving your boat, you ran over me as a swimmer, and it was your fault- say my injuries resulted in a claim of 1.5 million. Your boat policy would pay $500,000 and your umbrella policy would pay $1,000,000.
Now say you did not have the boat policy -- in that scenario, you would have no coverage for the loss under your home policy, no coverage under your auto policy, and because you did not have a boat policy, the umbrella would also not have coverage -- so you personally would be responsible for the full 1.5 with nothing to back you up ( your insurance company would more than likely also deny claim quickly and you would get to pay for your legal bills as well.)
So basically, a umbrella just adds limits to policies you already have. There are some exceptions to this, but those are limited.
Getting back to the efoil ( yes I understand what that is even if you need to talk slowly to me) there are several companies that will write primary coverage on them. Most "mega" companies will not touch them because there is not enough loss data to make a statistically reliable estimate of claims, and there are not enough potential policies to generate the needed premium. Insurance is always about having a large enough group of customers to make up for losses.
This issue is the same for ebikes as it is for the efoil. There are very few companies that will offer policies for ebikes, and generally those companies will only offer limited liability policies ( $15,000 to $50,000.)
So what do you risk going without a liability policy for that new ebike or efoil? Basically your financial net worth. The chance of loss is small, but if you are the "one" the financial devastation can be great.
again -- great question --