...fewer than cars, more than planes
And do you mean floating (in service) or lying under the water? Then, of course we would need to know whether you mean boat (any floating vessel) or yacht (pleasure craft)..or both.
Then we would need to establish whether it is a fact worth knowing!
Who cares?
As you have cross posted this question on 2 forums are you hoping to find out that there are twice as many?
Personally, i'd be much more interested to know how many boats there are in the world with owners who have so much money they'd like to send me some.
Please send private message with email address for contact and will send bank details etc by return.
If you check out http://ibinews.com (you can go there by clicking on 'IBI' on the bottom right corner of the nav bar at the bottom) and check out the Key Market Facts and Country Reports sections you'll see some estimated stats for the main leisure boating areas.