I have a nice casting deck on the front of jon boat but it obviously has added some weight to it.  Will spraying a can or two of that spray foam inside the front of the boat under the deck increase my boyancy?  My dad says no it won't it will just hold the front up if the boat starts to sink.  Anyone try this??

Thanks

Your Dad is correct.  The only time you'll notice the difference is if you sink the thing.

Well....it will but it won't, it's a displacement thing.

Don't use the spray insulation that is used for your house to fill holes and the such. This type of foam is known as open cell and it will wick and hold water. If it gets wet it will add weight to your boat, never dry out and cause fungus growth.

What you are looking for is "closed cell foam". Be warned it is more expensive and as far as I know it only comes in a two part "pourable" version, (not sprayed out of a can).

It will not help at all...unless like the other guys have stated, you are sinking.  The boyancy of the boat is already determined by it's size...you can not change that.