How do I get an elderly parent declared incompetent without his doctor's involvement?

I am an only child and the POA for my father. He is remarried and has a prenuptial. Several years ago he had some strokes that left him with some processing difficulties but not terribly bad. His current doctor is hesitant to declare my father incompetent, even though at 92 years old my father sold a large amount of his investments to allow him to buy a large boat he is unable to operate by himself. He tried to hide this from me but I found out about it before the boat was delivered. I didn't try to stop the purchase because I knew it would destroy our relationship. He is prone to falling, is in and out of the hospital every few weeks it seems due to falling and bleeding a lot, respiratory issues, etc. If his doctor will not declare him, can I take another route to help protect my father from himself? Our entire group of friends and family will support my taking away his driving and boating to protect others on the roads and waterways.