I doubt you'll find a clutch that will work as you want as they need to have high RPMs to get the clutch to engage, something more suitable to a race boat.
To get the boat to go slow enough, you will need either a small prop or a really well dialed in carb. I see a .12 or .15 as your best options for the low speed performance you're looking for as well as being able to get the boat up on a plane. Your biggest issue is going to be weight, especially if you're using one of those two small engines