Look at it this way: Your boat t-bones a classic Herreshoff yacht that was just restored and valued at 850,000. It sinks. The Coast Guard brings out a boom and pollution controls, and since they can't find you right away and the boats were in a channel, they also call in a commercial
operation to clear the channel. They hand you a bill for $150,000 for the cleanup.
You're now on the hook for a full million dollars. Your half mil policy pays half of that, and now there's a judgement to seize your assets. Got a house? A car? Anything in the bank? That's gone, along with 10% of your salary or income
for the next 20 years, until the second half of that million is paid. With interest.
On the other hand, if your boat is all you have to lose, and 10% of your earnings doesn't matter because you're judgement-proof...then a half million is enough.
These days? With hospital bills too? Sometimes a one or two million dollar umbrella policy, on top of a lower base insurance, can be the way to go. Of course if you don't sail near those expensive Yachts...even a quarter mil might be plenty.