I've bought boats on impulse and regretted it. Due diligence is key.
After you've viewed 20, 50 or more boats that seemed at first to fit your criteria, you'll get pretty good at knowing what's a good value and what isn't.
The key is being willing to walk away from anything that's not 100% perfect for you.
Here's a little secret: there are plenty of boats for sale out there. You'll never regret walking away from a boat, but you may very well regret buying one.
All that looking (and saying "no") just primes you for that one day, when you least expect it, the "right" boat swings into view.